Redditors in their 30's or beyond: do you feel like in some point in your life you started losing contact with some of your best friends? Why does it happen?

I'm in my mid-20's, and I notice that, for some reason, people start to slowly get away from some of their strongest friendships when they reach a certain age. At my parents age, it seems like a natural thing.

Does it really happen? Is forming a family the main reason? Is it strictly for practical reasons or does the very notion of friendship somehow change?