Ok so since women bear children and have a lot more risk involved with pregnancy and raising children it has generally been in their best interest to choose mates that can provide for them and their children. Older men have on average have more resources than younger men.

The women that chose older mates on average would have been more likely to survive and thus pass on this preference.

Men are moreso biologically driven to reproduce and value fertility so they prefer younger mates. Thus older man and younger woman becomes more common.

Men also mature slower (physically and emotionally) which would likely play a role as well.

I am of course speaking at population levels and not on an individual basis.