It is not so much age that interferes with BMT success ratio as all the other things that go on with a thal as (s)he ages. These are the areas that a BMT specialist looks at to estimate chances of success:
1. Number of transfusions - less is better
2. Chelation - higher is better
3. Damage to body organs, especially heart and liver - obviously, less is better
Hence, a minimally transfused, well chelated patient with intact organs has the best chance of getting a successful BMT. This usually coincides with younger thals.
Hope that helps.
Poirot