I would rather have a 21 yr old with the higher time. I say this on the assumption that the 21 yr old has probably been an aviation rat his whole life and is probably way more motivated and ready to learn. I also believe the 21 year old will be able to learn quicker. In my own personal opinion those that truly love the art of flying as most very young aviators do, their motivations and abilities are different from those of other career changers.