I've signed up following my uncle's need for one. Turned out I was a.match but an older woman was also a match, and it seemed they wanted to go in that direction since I was younger. Not sure if there was an assumption that she would be less likely to have issues in life requiring their own transplant.

Don't forget there are other good options for donation too, I have been on the bone marrow registry since I was like 25 because my boss's son got leukemia (sadly he didn't survive, but left an amazing legacy).