NBER research suggests unions correlate with lower relative employment for certain more vulnerable populations such as young and old.

There's also the risk the union basically exploits the young and old by driving their wage from shitty to zero (by setting them too high to bear for anything but the most able bodied middle age). This pretty much mimics my experience younger working construction; union gigs set my wage at '0' by making me the outgroup. Being exploited by middle age union workers is bad for longevity imo.