This question has haunted me for a while, but I always thought there was no info about it at all. We do know his age: the info booklet of Dawn of Sorrow states that he is 56. Assuming it is exactly one year later than Aria of Sorrow, he was 55 there. The events in Aria of Sorrow are based on a real-life predicted eclipse: a total solar eclipse will be visible from Japan on the 2nd of September in 2035. This tells us that Julius was 55 on 2/9/2023.
We can’t know for sure how old he was during his battle with Dracula in 1999, which happened on the 11th of August that year, the date on which a total solar eclipse was visible above Europe. However, it is likely he was 19, because then he would be at the same age as Richter when he fought Dracula in Rondo of Blood.
The wiki states he was born in 1980, but that does not take his birthday into account. If his birthday is before August 11, he would indeed have been born in 1980. But if his birthday is after August 11, he would have been born in 1979. The same goes for him being 19 on the 2nd of September: if his birthday is before the 2nd of September, he would have been born in 1980, if his birthday is after the 2nd of September, he would have been born in 1979.
What does this tell us? Not that much. If we assume he was born in 1980, then it means his birthday can be anything between January the first and August the 10th. The only thing we can tell for sure is that his birthday is not between the 11th of August and the 2nd of September.
Fun fact: there was also a total solar eclipse visible on the united states on august 21, 2017. This is Soma’s date of birth. This explains why he was introduced as an exchange student in Japan during Aria of Sorrow (which is a bit weird since Mina is referred to as his ‘’childhood friend’’): because of the eclipse thing he would have had to be born in America.
