Is that even a serious question? It's literally biology. Humans are in their prime between like 18 and 24 years old (just made that range up so could be +/- few years but likely close). Same concept as why a fucking tomato will look worse 4 days after you bring it home from the store. We are organic beings. Organic materials degrade. Just accept it ;-). There is more to a relationship than looks.

You may wanna bang a 20 year old when you're 40, but would you want to date/marry a 20 year old? I certainly fucking wouldn't.