I've never read your fic, but in general, if you're 18 or 19, even if you're making your Inquisitor 38 or 39, chances are you don't have the mentality and life experiences of a 38 or 39 year old, and therefore can't write an entirely convincing 38 or 39 year old.
So even though technically I'm not reading about a teenager, I still am, in the sense that a lot of the reactions and world views are of a younger person.
There's a reason they tell beginning writers to write what they know.
Again, is that true for every author and every fic? No. But a lot.
Although now that I think about it, I'd actually read a well-written fic about a 19 year old Quizzy having a May-Uh....September
romance with Cass, if the author could make it believable.
The mentality one has isn't determined by how old they are (it's certainly influenced by their age, but age is not the sole deciding factor). One does not need to be something to be able to portray that something. On paper, anyway. On screen or on audio, it's a bit of a prerequisite to actually be the age of the character, or at least close enough to look or sound like them.
If we locked it down to "well, I'm a straight, white, 35 year old male who's been working in a cubicle for the past decade, so I should only write about that", we'd be losing out on a great deal of valuable art and potential insight on different subjects. A younger person's outsider perspective can be just as valuable as a veteran's (arguably more so in certain situations, since it's less likely that the younger person would be as jaded or disillusioned as the older person).
Of course, that's not to discount the experienced people and their view on things. I value them just as much and appreciate what they have to say as well.
It also helps that I have a vocabulary that includes words longer than 5 or 6 letters, unlike most people my age, so I can at least pass as smart. One of the more common bits of praise I've seen for my pieces is that I don't "dumb down" (for lack of a better term) the writing with caveman language. Like, I don't purposely use big words to be pretentious, but I don't write under the assumption that the reader is a 5-year old. Indubitably and impeccably.
I've actually been mistaken for being older than I actually am from my writings, but I just laugh those comments off.
edit: what is AO3?