3 weeks old, seriously. At least wait for 4 weeks. But anyway, just get one more for him to be with. When I got my other hermit it didn't seem like they were together but it worked. Also just leave the shells in the tank. My hermits inspected the shells for about 2-3 weeks before they decided to change.Tanks 56l and only just over 3 weeks old.
I was going to wait a little longer before adding more CUC.