I don't have a pic but Ink from Moetan. Everyone says she looks like a grade schooler when she is almost finished highschool. Also, Konata from Lucky Star. She looks like ten when she turns 18 in the anime.
Mimi from Mnemosyne looks around 15-16 but she's actually at LEAST 917 years old and was actually in her late teens when made immortal 17-18. They never give an exact age for her and never say how old she was when she was made immortal. Rin is the same way x3 but she's at least 1,025 from the peeks they gave at the clothing in her flashbacks.
The picture doesn't want to subir either so whatever.... I'm not in the most fabulous of moods right now anyways...
Yu Chigusa(a.k.a. Johannes), from the light novel titled Qualidea of Scum and a oro Coin, has appeared in Qualidea Code, an anime set at least 30 years after, and she doesn't look much older than in "Scum and a oro Coin."
Mephisto is literally as old as time, but he looks only to be about in his mid to late 30's. Probably because that was the age of his host body when he obtained it... And Amaimon looks like a teenager when he's thousands of years old.