I believe a lot has to do with the culture and people being too much online which is bad for people health

Also if a person has the right partner they will settle down and have children it's just that we are living a lot longer and people at 19 and early 20s

Probably won't be looking for a wife or a husband it's like a game of musical chairs and as we get older more and more chairs disappear

As we get older as well we know our hobbies where we want to go and holiday and it gets harder to find a partner that shares those interests

It is better to find a life partner/wife/husband when young because both parties can grow and change together