I'm curious because I'll be having these issues with my own kids in a few years, + I thought would help out this dad because there will always be some good material in this subreddit.

I'll start: One month before 16th birthday. While I believe that another 16 y.o. in the right relationship could have been ready, I was not. Lots of pressure from a 19 y.o. controlling boyfriend. While I was a roiling fit of hormones, my interests and sexual desire had not matured & I had not figured out how to be assertive in interpersonal relationships. I wish I'd figured out my own body & sex more rather than thinking sex with someone else was the only place to go (& I'm talking masturbation + just general better knowledge of my body). I wish I'd realized that you have to be pretty aggressive with barriers towards teenage boys to avoid having sex with them.

Next time, I was a senior in high school, & was absolutely ready. I've often thought senior year of high school is the best time. You're somewhat mature, your body has developedm you're not in the midst of the chaotic upending experience of starting college, & you're most likely in a relationship with someone you've known several years, which is less likely in early college.

I'd tell my younger self about how there's no need to wait for marriage, but that it's better to wait a bit longer, that tons of research shows that your judgment matures significantly over a change of a few years so it's not about an infinite wait but 12-36 months matters. And in the interim to be less sexually inhibited, to be more assertive about what felt good & comfortable for me physically & emotionally, to ignore any chatter re: blue balls & prick teases, to dump any boy resentful of stopping at heavy petting, to read more books about sex, & that it is perfectly valid to stick at oral sex for a few years as it is extremely fun & has less complications.

Edit: & I'd also love to tell my younger self how effective it is to frankly & bluntly tell a boy at the beginning of the evening that I was happy to fool around but we were not going to have sex. God knows the young ones are falling apart with curiosity & don't know what's going on either.