I think that Darla was probably in her early 20's when she was turned. We only know that she had been a prostitute and was dying of syphilis when she met the Master. As for her appearing to be a teenager in Season 1, that could have just been makeup, hairstyle, and the way that she was dressed. As far as that goes, they never said that the boy that she was with actually thought that she was a teenager, and she obviously wasn't attending classes at the school. Probably, he just thought that she was a college student who liked younger men.
18 seems a bit young to me. I'd say Benz was probably about 23 when she played the part (I'm guessing without cheating and looking at her birth date). Because of the way Darla acted it gave me the impression she had a bit more life experience. The school girl costume in the pilot was just the cover to get her pray I think.
Darla could obviously pull of the 'younger than her years' look that is heavy coveted. She coulod pass for 16 or a young sophisticated lady-type in her twenties (cloder to her actual age when she died I'm guessing)
I like to think that Darla's school girl outfit and predatory behaviour nicely subverts the incredibly creepy future trend of having vampires attend high school over and over again, like that Edward-guy from 'Twilight'.
I assume that "Julie" was a glamour, and Darla's true form was the grotesque visage. I also assume Vampires could masquerade as people of any age, if they wished, although, I don't know by what mechanics. For all we know, Vampires are supposed to look timeless rather than youthful, and Darla fit in because she was wearing a dress.