Why are the men usually older in heterosexual relationships?

I see this a lot in Reddit and Irl and I can’t wrap my head around it. If their something going on between a boy and a girl, the boy is either older or the same age: I’ve almost never seen a situation where the boy was younger. I’m not saying it’s a problem or anything: Usually it’s just by a year or few and isn’t usually pedophilic or anything: I just find it odd