Why is a 19 year old liking a 16 year old frowned upon in our society?

I can't understand this. An 80 year old could marry an 18 year old and while society would think it's odd, people would understand. But a 19 year old dating a 16 year old and it's not OK. Can you guys help me out here?

And before you ask, this is a throwaway because a lot of my friends know my main account and I really don't want to take a lot of shit for this.