As you know, many of my stories feature underage main characters. I have a few questions. Are either of these appropriate.

1. Just saying Brittney, A teenager and never specifying an age.

2.  Courtney, 18, who looks young for her age.

Thank for the welcome back, I appreciate it.O

I would state their age and describe what they look like.

18 or 19 is fine according to the rules. Also from a personal standpoint those ages are perfectly fine. I’d make the first one 19.

Don’t know if this is still valid, but it used to be ok for the main character to be underage as long as they weren’t engaged in any sexual activities. Personally I like to play it safe and state that characters are above 18, then describe how they look.

Probably stating age of 18 could be fine, though there would need to be an explanation for a similar sense of virginal vulnerability, innocence and ignorance.
Might work for a practicing novitiate living out of a convent, or maybe sort of a rural, hick town setting, where due to a prudish upbringing watching farm creatures mating is all the gist a female character could get.

A bit like a gender swapped H.P. Lovecraft.

A medieval setting could work, where the mysteries of reproduction could be chalked up to "magic" and the fae folk.