Of course we are generalizing, but when I was young I saw lots of jealousy manifested and I was the jealous sort myself. I think it was because relationships were fresh(untested) and hormonally intense .
I’m 57 and I can’t recall myself or others that I’ve associated with being jealous. Our relationships are either secure or perhaps stale and unvalued. There’s even more flirting though, but most of understand the extent to which it should be taken seriously.