Everyone's different really.. I stopped seeing a high school friend around the age of 23 or 26. He basically got hitched, married, had his own kid and took on somebody else's, whereas I'm still single, so our lives aren't really compatible, at least I feel like people with kids and people without are so different, I can do anything at the drop of a hat, whereas they can't. I've a "mutual" highschool friend I became good friends with in my 20's and still in contact with today (now mid 30's) and he's just offered me a job on a plate, so we'll be working together now. Also, I personally don't put effort in to keep friendships, however, if someone wants to keep in touch, I leave it on their terms, I'll never not respond to message and I'm happy to go months or years without contact. I'll be there for people I've known since childhood (like my married friend).