I don't have a first hand account. My oldest YF whom I am still in touch with is still a teenager. Our relationship has definitely evolved over time, but that's not what you are asking about.

I do, however, have a second hand account. It comes from my oldest bl friend (oldest in terms of how long we have known each other, not how old he is). I have known him long enough to have been the first person he told about meeting what would become his yf back when the boy was 11. The boy is now an adult. I really can't say a lot about how their relationship has changed because we haven't stayed in touch as much as I'd like over the years. But I can say that they still comment on each other's Facebook posts and pictures of them hanging out occasionally crop up. It seems the only thing keeping them from continuing to be best friends is just the direction their lives have taken. My friend is doing this whole 'travel the world' thing while his YF has chosen to get married and settle down.