I think as you get older it becomes more difficult to maintain those old friendships you used to have, whether because of distance, growing apart or just a lack of commonality and moving on. Also when you set up home, or in a relationship or have children, you become relatively self contained. The need to get out and make friends and be a socialite is relatively diminished because you are content with what you now have.
Of course there is an element of social interaction and friend-gathering, but at a much lesser extent as you age. This is quite noticeable on sites such as Facebook etc, where the younger generation will have 2,000 or so fair-weather �friends�, whereas the older generation have 20-30 good friends. I often believe that people my age with many many friends on these sites are desperately trying to hold on to their youth, or look popular - a sign of lonely arrogance??!
I hope people didn�t think I was demeaning the loner in my post above, as much of what I have written describes me.