Yes and you're not wrong to feel bad about it, it's pretty hurtful whenever it happens, regardless of age.
My experience is quite recent:
I have two different groups of friends, most of them have at least one child now.
With one group, who don't live particularly near to me, I do get invited to "child friendly" events and I enjoy this just as much as going to the pub, possibly more as I'm not really a fan of a hangover these days! I love my friends and their kids, despite not having one of my own (we are trying but we're are a little older in starting than everyone else, life just got in the way) and it's nice to spend this time with them.
With the other group, actually my oldest friends, I don't get invited to group/child friendly things at all and the meet ups without children are very rare. I hadn't really thought about it much until something was mentioned by one of the group; I had invited them all to something I was organising and they didnt reply until about 6 weeks later. I know life gets in the way and I can be bad at replying sometimes myself so hadn't been to worried about it, however it turns out this friend had chased them all to do so, telling them it was poor manners not to respond. I guess that was the bit that hurt, that I wasn't important enough to them to type a quick "thanks but we have something on" to.
I've not tackled it yet although I have told my friend that I was a bit upset and she has been understanding.
I think I need to branch out and meet some new people (who live closer to me) to become friends with, perhaps take up a new hobby to meet them. Perhaps that's something that you could do OP?