Age-gap relationships and the glorification of them isn't healthy...
Yes, those kinds of relationships usually worked out in the past because they were merely business deals - trading your daughter for livestock, some land, money or a title was the norm, and it didn't really matter if said daughter was happy or not...what mattered was that she served a purpose to her family and to her husband.
Nowadays usually no one is forcing the youngster to go into an age-gap relationship with a significantly older person. They do it because it sounds like a good idea, and within romanticizing that idea they seem to have a lot of suppositions that actually a lot of the time turn out to be false - hence they get disappointed and things, surprisingly enough, just don't work out for most of these relationships.
Here are some of the pitfalls in the suppositions young people make about age-gap relationships:
1. Just because they're old/er doesn't mean they're mature.
2. Just because they seem rational and well-mannered doesn't mean they aren't going to manipulate you and use you.
3. Just because they are with you doesn't mean they take you seriously.
4. Just because they have numerous life experiences doesn't mean they've learned from them.
5. Just because they're reached a certain age doesn't mean they're stable and independent in life.