Older pilots have a lot of advantages. One of the most important skills is life experience. They are usually more educated and take less risks. When I walk into the cabin I always look into the cockpit just to see the age of the pilots. A gray hair captain makes me feel good and a gray hair FO next to him makes me feel safe.
If I were to choose between a 250hrs 20 years old and 250hrs 30 years old, I would go for the 30 years old just because I know that he has more life experience and he will take less risks.
I agree, above 40 the health issues will overcome the experience advantage compared to a 20 years old.
In the US it is a fact that you will never get into the majors before 33-35 and 2500TT under your belt with 1000 turbine.