Gotta wonder if she's "of age" by our modern standards or by the standards common to medieval settings. Because, honestly, she doesn't behave with the emotional maturity I'd expect from a teenagers, to say nothing of a young adult. The way she's written, she struck me as being in the 12 to 14 age range, top.
I mean you're never going to get a straight answer in that department for obvious reasons.
All the game says about Feremina is that she is nubile, "of age", and Yarra could sense
some sexual energy in her, but even she wasn't thrilled about the idea of adding her to Simon's harem yet ("would feel like picking a fruit before it was ripe"). I do think the game is doing the best practice for this kind of scenario by leaving the writing ambiguous. I don't remember if it is something that is actually in the game or information from outside of it, but Nalili actually being 18 years old (almost 30 in "succubus" years) never sat right with me, even with the "spoiled princess" background.
Either way, I see no value in Lilith's appeal being "1000 years old lolita", and not really due to any particular distaste for the trope but mostly because I feel that niche is already kinda fulfilled by the halfling characters? I feel like the appeal of "cute-looking small women that are actually sexually active and naughty" is already done pretty well by Sarai and Wynn, what would be the point of adding another one through a chaos entity as if it was such a
kinky premise?
I could maybe see some potential to it if the idea is that Lilith is willingly using her form to try to rile up Simon, but I feel that would just go down like the Trin scenes (overtly try to seduce Simon into a ridiculous scenario, Simon's too gigachad to be aroused by "wrong" things, at most he sort of plays along with it by not shutting the entire thing down).
EDIT: like Fulminato said, if you wanted to follow an absolute "18+" rule, since Feremina is a halfling she could be 18 years old, just that 18 years old for a halfling is not as far ahead from their puberty as it would be for a human.