To me, a serious relationship is one where both people have the ability to influence each other's life, feelings, mind and/or circumstances. It's not necessarily that one has the financial resources and connections to improve another one's material and social status. But in a serious relationship someone can affect who you are as a person -- open you to a new level in some way. He/she changes you or grows you in some way.

Or, it could be love. If you love someone, it changes you.

IMO there's a definite division between people under 25 and people over 25. They are still in a clueless cocoon of innocent naivete that clings to very young adults like baby fat. It's hard to take anything from someone that is still living in a world of their idealistic expectations. It's hard to not unintentionally lie to children who believe everything you say... you have to go out of your way to make sure they really understand the things you say, and are not just projecting imagination and infatuation into everything.

And the age is a generalization. I'm sure there are 23 year olds out who had unusually mature experiences, there who can teach immature 42 year olds plenty about life. But IMO those are the exception, not a rule.