I Don't Think I Belong Anymore

I've been off with a group of teenagers for a workshop this week. I have really enjoyed being around them. I also teach high school and thoroughly enjoy my job but I"m beginning to have my doubts. Several years ago I would have been right in the middle of what was going on with the young people in my life, especially if one of my children was involved. I don't know if it is because I don't have a school age child anymore (I kinda think that is it) or it I'm just too old but this trip hasn't been as much fun for me as usual. Now just two months ago I was in the big middle of my son's high school graduation and had a blast but now I kinda feel like I don't belong with these kids anymore.
The workshop has been great, the kids have been great and for all practical purposes this shuold have been a good trip but, I have felt out of place. We took the kids bowling tonight and I love watching them play but when I decided to sit it out, they didn't object. It's almost if older people are invisible. PLEASE don't get me wrong, these are very well-behaved kids and not a one of them would purposefully hurt my feelings but I see this happening in a lot of areas in my life. Somehow I don't think I have value anymore since I don't have a child to be involved in things with.
What I can't figure out is whether this is a mindset, a reality, or a little of both. All I know is, I don't like it. If anyone out there can help me pinpoint the problem, let me know. Thanks for listening.