This blog is meant to confirm the ages of characters.
List A-Zeoticus, Venelana, Misla, Lady and Lord Phenex, and Lord Bael
These characters are over a 1000 years old based on a quote said by Azazel in Volume 21.
"Do you need to bear the name of ‘Lucifer’ to such an extent? I always thought that. Sirzechs was still quite young as a Devil, he was a young man who hadn’t even lived a thousand years yet."
Volume 21, Khaos Disaster
This means the age to become an adult in Devil years is 1000. All known adult Devils who are mentioned above then are 1000+ years old.
List AA-Zekram, Azazel, Baraqiel, Shemhazai, Penemue, First Devil Heads, Mephisto, Kokabiel, Gods/Goddesses, Armaros, Sahariel, Tamiel, and Rizevim, Gabriel, Michael, Raphael, Uriel, Metatron, Sariel, Sandalphon, Raguel, Remiel, Fenrir [and others similar like Ddraig and Crom Crauch], and Raziel.
Zekram is around 10,000 years.
Rias’s Dad says to me like how a father would say to his son.
"Ise-kun, remember it very well. That man is the actual man standing at the top of the Great-King faction. You can say that the political influence he has is greater than Sirzechs. After all, he is a man who has been watching over the Underworld ever since the beginning of Devils. The years he has lived are far different to us. If the Four Great Maous such as Sirzechs and others are the light of the Devils than that man called Zekrum Bael-sama is the shadow of the Devils."
Volume 18, Life 2
The Fallen Angels and Angels listed have are too since they were the first of their kind; same with the other progenitors of the 72 pillars and Extra Demon clans. Gods/Goddesses and mythological beings like Fenrir have at least lived for 10,000 years. Rizevim is the only son who was alive of the original satans so he had to be close to 10,000 too being the direct son of Lucifer; just not as close as Zekram. The Three Factions all come from the same religion so the approximate maximum age of Devils which is 10,000 is the same for Angels and Fallen Angels too.
List B-Sirzechs, Ajuka, Serafall, Falbium, Grayfia, Euclid, Katerea, Shalba, Creuserery, Sun, Zhu, Sha, and Xuanzang
These characters are over 500 years old. Zeoticus' Bishop, Heinrich Cornelius, was reincarnated in 1506 so Ajuka who created the Evil Pieces, had to be alive at that time for Heinrich to have been reincarnated. Sirzechs, Ajuka, Falbium, and Serafall grew up together and Grayfia, Euclid, Shalba, Creuserey, and Katerea pretty much come from the same generation. Zhu, Sha, and Xuanzang are over 500 years too as Journey to the West where they are from takes place in the early 16th century. Sun's story originates in the Song dynasty which was from 960-1279 so he is around a 1000 years old.
List C-Issei, Yuuto, Xenovia, Irina, Asia, Saji, Matsuda, Motohama, Murayama, Aika, Tsubasa, Tomoe, Momo Hanakai, Katase, Reya, and Tadami
These characters are all/will be 18-years of age. Issei while he was a second-year in high school, was 17, so in the third year he will be 18. So that means all characters previously in the second year who have moved up to the third, are all going to be 18 too.
List CA-Koneko, Gasper, Ravel, Ruruko, Ouryuu, and Millarca
These characters are all/will be 17-years of age. Since second years are 17 has proved by Issei then these guys will be the same age.
List CB-Le Fay and Bennia
These characters are all/will be 16-years of age. If a second year is 17, then a first year will be 16.
List CC-Rias, Akeno, Sona, Tsubaki, Kiyome, and Loup
These characters except Loup all are/will be 19-years of age. They were previously in the third year so they were all 18 and now that they will be starting their first year in college, they will be all 19.
Loup is/will be 20 given he was in college first before them.
 This table shows the age ranges in Japanese education.
List CD-Tosca and Kunou
Tosca is 18 years old like Yuuto but appears to be 12-13.
--Kiba was taken aback, his eyes widened. Then, he could not help nodding. Strada turned his eyes towards his subordinates. Then, a young girl emerged from a group of warriors. It was a girl around twelve to thirteen with white hair tied into twin tails. As soon as she saw Kiba, she frantically covered her mouth, and tried to hold back the tears which were gushing out from her eyes.
The girl asked that question. Kiba was in shock as tears streamed down his cheeks.
“……! I-Is, h-how would…! You’re Tosca…?”
Strada said to the speechless Kiba and us
“Only this one girl with a strong barrier type Sacred Gear survived. It was discovered after the experiment, and Valper was unable to do anything. Although the user was in a state of suspended animation, it wasn’t possible to be released, and researchers had no choice but to place her into a deep hidden chamber within the facility. And she herself, when Valper was expelled, was found during a search of the facility, but we also were unable to release the barrier. However, after the alliance, we were finally able to release that barrier by using the Fallen Angels’ technology.”
…Still alive! One of Kiba’s comrades was still alive! And that person was actually recovered by the people of the Church! Strada said
“Because she was held in a state of suspended animation whilst inside the barrier, her body’s growth stopped, and it looked like she was very weak. Thus, bringing her to this country took some time.”
The girl walked to Kiba’s side, and then extended her hand out to caress his cheek.
“Isaiah, you’ve already grown up…. …We were clearly the same back then.”--
Volume 19, New Life
Kunou is/will be 12. She is starting middle school and to join as a first-year in middle school, that person has to be 12.
--"Oh, Kunou will be starting middle school at Kuoh Academy in the coming year. The preparations have already been made.” —!? The sudden news left me at a loss for words! It was too unbelievable! It was so unexpected, that something like would happen!"--
Volume 19, Life 0
The current year ended in Volume 19.
"At the end of the year, I was at a certain small stream holding something like a fishing rod. ……Although I do enjoy fishing…even I don’t feel like fishing right now."
Volume 19, Rudra
List D-Valerie and Rossweisse
Valerie is/will be 21-22 years old. Issei stated she is 3 or 4 years older than him when he was still 17, so she was 20-21 and now that he's he 18 she is now 21-22.
--She appears to be three or four years older than me. A shapely face. You may call her a beauty. Instead of having a crafted beauty of that of a complete doll like Elmenhilde, she has a beauty of a human appearance.
……She probably carries both the beauty of a Vampire and a human since she’s a half-blood--
Volume 16, Life 1
Rossweisse is/will be 18-19 years old.
"Of course….. Despite my appearance, I have already graduated from school in my homeland by skipping grades. Even though I’m still young in age, I am qualified to teach students.” Seriously? She doesn’t appear to be older than us at all, but she chose to become a teacher."
Volume 7, New Life
"Her setting age is 17 to 18. Even though she is young, she chose to become a teacher instead of being a student. So that will make it compulsory for her to go to the school trip. This is the birth of Rossweisse-sensei!"
Volume 7, Afterword
List E-Tobio, Dulio, Vali, Suzaku, and Lavinia
Tobio is/will be 22 years old. In Slash/Dog Tobio is 17 as he is a second year high school student. DxD takes place 4 years later and another year is passing so he was 21, but now he will be 22.
Saying so while standing smiling at the front door was a semi-long haired girl. She’s a classmate of Tobio in the second year of the same high school. She’s also his childhood friend, Toujou Sae. Her smile was mischievous.
Dulio is/will be 21-22 years old. Issei stated he is three or four years older than him.
Sister Griselda sits on one of the table chair, but I’m concerned about something else.
……The two men that Sister brought with her. One of the men in a priest’s outfit is a handsome youth with blond hair and green eyes. Is he three or four years older than me?
While I start to come up with assumptions, the man who is dressed as a priest changes his expression and greets us.
“Howdy? Nice to meet you all, Devils of Kuou academy. I’m called Dulio Gesualdo. Let’s get along from now on~.”
Volume 15, Episode Issei.5
Vali is/will be 17-18 years old. He is 12-13 in Slash/Dog.
Suzaku is/will be 22 years old. In Slash/Dog she was 17 so now she will be 22.
Regarding the age of the young girl——Himejima Suzaku, she was seventeen as of this year. Her appearance was fitting given that she was one of the granddaughters of Himejima Suou’s other older sister. As of this year, she had been the one talented woman from all of their relatives who had inherited the sacred beast ‘Suzaku’ that was governed by the Himejima. No, in recent years, the ones who had inherited the sacred beasts of each of the respective Five Principal Clans had been remarkably young. Based on this, the efforts the families had undertaken to bring in good blood, they appeared to have borne fruit.
Slash/Dog, Final Chapter
Lavinia is 22 like Tobio. She is described as being about the same age as him in Slash/Dog.
Appearing out of the darkness, a strange looking blonde girl stepped into the light. Also, the reason she was strange looking, she was dressed in a cloak and a pointed hat. Though she looked foreign, she was dressed as though it was a witch cosplay. He found himself surprisingly fascinated by her most noble appearance. Although it was hard to judge with foreigners, could she possibly be……about the same age as him.
Slash/Dog, Chapter 2 Part 3
Millicas is/will be 6 years old. He was born a few years prior to the series which is 2-5 years, but he was not born 2-4 years ago. Given his appearance, he is not a baby [2-3 years] and he is not the height of a four year old boy so he has been 5 years old this entire time, so now he will be six years old.
List G-Rudiger, MacGregor, and Souji.
Rudiger is over 600 years old. He is said to have been alive in the 15th century.
MacGregor is over 200 years old as he was born in the 1800's and it was never stated like Souji that Sirzechs reincarnated him sometime after his death which was in 1918.
Souji is over 150 years old as he was reincarnated by Sirzechs some time after his death which was in 1868.
Vasco is 87 years old.
"His Eminence Strada is already eighty seven years old…"
Volume 19, Life 1 Part 1