This is what it said on Marrow.org
Q: If I'm over 60, why can't I join?
A: The age guidelines are in place to protect donors and provide the best treatment for patients:
Donor safety: As one ages, the chances of a hidden medical problem that donation could bring out increases, placing older donors at increased risk of complications. Since there is no direct benefit to the donor when they donate, for safety reasons we have set age 60 as the upper limit. It is important to note that the age limit is not meant to discriminate in any way.
To provide the best treatment for the patient: Research shows that cells from younger donors lead to more successful transplants. That’s because younger donors produce more and higher-quality cells than older donors.