Feel free to not hire someone because they say that they're going to use obsolete methods, or because they say they're going to take leave in excess of what's offered, or because they say they're going to resign in short order. Just don't use old age -- instead of statements like these -- to make assumptions!

When someone is too young, however, there can be some guaranteed deficiencies that don't require any assumptions in order to know the requirements can't be met; for example, if you are looking to hire someone to create and execute a 10-year plan with prior experience doing so, it's impossible for a very young person to be qualified.