I am 17, and I have a 15 year old sister who is Autistic. I also come from an EXTREMELY Catholic family. I never got a sex talk - I straight-up asked my dad what sex meant when I was 9 or 10, and he gave me some very unhelpful answer about a gift that God intended to be shared between a man and a woman in marriage. I, however, had enough resources like gurl.com and, you know, friends with older sisters to eventually get the full picture. My sister does not.
Katie knows about menstruation and deals very well with it, but at last check she barely knew what her parts were and she does not appear to be receiving any meaningful sex education in school - that's my school district through and through. But Katie is physically mature, and I'd bet almost anything that she's experiencing age-appropriate sexual feelings.