I'm 40. I ran my first two marathons in my mid-20s with a couple friends, and trained with them. Since then I've probably run with a partner two or three times, all of them last summer with a friend when we both worked in downtown Chicago. He's a 3:4x marathoner and I'm probably a 3:08-12 guy at this point, so I'd wax him pretty good on the last mile or two of 10-11 mile runs, but we still pushed each other.
This is one of those things, though, that people here seem to think is weird, like wearing headphones or running with a full-time job or family, that is actually normal. I get up at 4, hit the door at 4:30 or so, alone before work, and that's that. I wouldn't even know who to run with if I wanted to.