Those meetups where everyone is decades older than you? Many of them have children that might be your age. Make friends with both. Some of my best adult relationships are from older folks with similar interests introducing me to their children.
I do agree about the general uselessness of Meetup.com though.
1. Meetups geared towards a demographic I'm not part of (the biggest occurrence is 50+ meetups)
2. Meetups that are a shill for some MLM, other scheming
3. Tech meetups on subjects that dont particularly interest me
4. Bar / bar hopping meetups, but everyone is decades older than me