Complex sentence: Early onset Alzheimer's can develop in people who are in their 30's or 40's, but that is extremely rare; the majority of sufferers are in their 50's, or early 60's.
Split into simpler sentences: Early onset Alzheimer's can develop in people who are in their 30's or 40's, but that is extremely rare. The majority of sufferers are in their 50's, or early 60's.