Which is easier?cat or GRE?is there any age constraint for these exams?

I assume you meant the MCAT. Generally, the MCAT is seen as being harder than the General GRE exam because the subject matter of the MCAT is college courses, whereas the GRE is essentially like the SAT. The GRE subject exams are, in general, quite difficult.

And I don’t think there is an upper age constraint. I don’t know if you have to be an adult to take them though. If you are a minor you might need parental consent.