Humans produce eggs... not seeds. Sperm would be comparable to pollens.
And sexual reproduction is possible much earlier, i think at around 11-13 or so? Basically once girls start getting their periods. And even a 18 year old person is not "ripe" in that sense, since they're far from being done develoving and start "decaying". Puberty ends after around 22 or so. I think.
tl;dr: Your analogy is not only weird af but also false.