Im not sure if its exactly a requirement but priests have to have a degree and usually an advanced degree. Granted many get it in divinity but they also have to do seminary which can't start until age 25 and it is at least 5 years. So priests basically start out at age 30 and technically a pope could make one a cardinal right away in practice the average age of a cardinal is like 70. Popes can be drawn from anyone but are usually biships. Bishop are required to have a doctorate in divinity and be over age 35. They are also expected to be known for all sorts of good qualities. So figure to be known enough you can add 10 years being a biship. So for all practical purposes its very doubtful someone would be name pope before age 50 and they have usually been 60's or 70's.