Being best friends requires a mature level of real compromise, dependability and acceptance. A person that you can count on to be there physically and/or emotionally, based on the situation. Someone you can count on to tell you the truth, and give you good advice and support, delivered in a caring manner. When they fall short, best friends are people who can hear about it, really apologize, and do better next time. I didn't develop deep friendships until I was in my 40's...good acquaintances, sure, but not people I could really count on to be helpful when the sh** hit the fan. It's a very special thing to have, that requires proximity and time to become a valuable relationship.
I know many couples who love each other and are together, but they aren't best friends. I don't know how they maintain that level of intimate relationship without a deep friendship as well.
I think true best friend relationships are just as likely, or even more likely to occur for late-middle aged and older people.
A really special significant relationship can develop for someone in these age brackets from a best friendship. It's a more deliberate thing, with more to hold it together than just desire.

And if the above was "too much," you can always fall back on this sure-fire gauge for determining whether or not a deep and lasting, intimate relationship is possible: Just ask yourself..."Do I care enough about this person to pick his/her hair out of the tub drain for the rest of my life?"