When my step brother got married we were invited, including D, who was a 6 month old. We didn’t go because I was a SAHM at the time and we didn’t have a ton of extra money for travel. We did send a gift. It didn’t cause any hurt feelings at all.
The kid thing is tough at weddings. I know people who don’t take their kids even if kids are invited because they want a night out and they want to enjoy the wedding and not have to keep an eye on their kids. Another time when our kids were little the 4 or us were invited to a co-worker’s wedding and just H and I went. It was probably for the best, it was a church wedding and the only other kids there were all teenagers. We probably didn’t stay for entire reception, but it was nice to have a night out as a couple.
Later on both D and S were asked to do readings at their cousin’s weddings and D was asked to help with the guestbook and hand out programs at a family friend’s wedding when she was 13. In these cases, even though they were teenagers they were a part of the wedding and it was family and close friends.