I'm a grad student now, so most of my cohort is around my age, but I was also a little older during my undergrad, and I will say this. You won't have the SAME experience as an 18 year old, but frankly, I was perfectly okay with that. (No offense to 18 year olds, I know many that are awesome, but many are also interested in issues that I really don't care about very much at all). I still had a great experience. There might be different avenues to pursue in order to find yourself a good social circle (if you're interested in that) so all I can say is get involved, if at all possible. Join a club, or a choir, or whatever works for you! Not all of the clubs might end up working for you, so just try a different one. I'm involved with a couple, where the age range is quite diverse, not grad student focused at all, and I am having a great time. I've also been a TA and have had several students that were late starters and they were often my favourite students!

so, tldr, I hope you manage to find your groove, and really enjoy your experience :)