I think a lot of factors that come into play. I hear COMPLIANCE to Iron Chelation is the most important one. For your son's age or for younger children, NOT doing Iron Chelation is NOT an option. We as parents give it to them with heavy heart and they take it. But, I think as they grow up into their teenage, they have things in their own hands and teens(as we all know) are less COMPLIANT. When they do NOT comply, the damage starts to be done.
Another factor that could be into play is that Thalassemia was quite unknown 30 or 40 years ago and there was NOT much known about Iron Chelation and why it should be done(sadly in some parts of the world, that is the case even today)
Blood Transfusion also carries the risk of being infected by viruses, which the modern testing of blood has dramatically reduced
Other members might have thoughts to add