As established in the 1970PT storyline and explained here
Barnabas was supposedly 25 when he was cursed. Though the topic also mentions the caveats of why that might not be so.
The interesting thing is I remember Frid commenting more than once that it was fun playing Barnabas as a "youth" in 1795. I thought that was an odd choice of words at the time - but also at the time I'd completely forgotten about the 1970PT gravestone.
As far as the Shadows on the Wall bible goes, at the start of DS:
Victoria is 20 (and that tracks exactly with one of the '66 promos that said that "20-year-old Victoria Winters has come to probe the mystery of her past.")
Maggie is 23
Carolyn is 17
Elizabeth is 56
Roger is 36
Burke is 32
Joe is 21
Sam is 53
Frank is 27
As far as Julia goes, it's almost anyone's guess what age she might be if Barnabas is supposedly only in his mid-20s because quite obviously the age of the actors may have very little to do with their characters' ages. Though we do know that Julia would have had to have completed medical school, her internship & residentcy, etc., so she could easily be 10 years older than Barnabas.
It's also anyone's guess how old Professor Stokes might be.
And one would assume Angelique is close to Josette's age - but who knows...