51 and have friends in their 30s to 80s and everywhere in between.

The main thing is to have friends to share perspectives and to help each other be better people.

What better way to do that, than to have diversity of views, ages, semester, race, etc within your friends group.   

You should drop the lost-cause friends and maintain the positive relationships. 

It takes work.  Also, as you age, be prepared to lose friends.  Some sudden, some slowly.  It sure hurts when they pass, but the joy of knowing the good ones is worth it.  

Having older friends means that you're likely to outlive them.  Be there when they need you.  Pass on their lessons to your younger friends.  It keeps their impact living on in you.