You talking to make a career out of it or to enjoy it? If it's for enjoyment you can relate it to porn - when are you too old to enjoy porn, when you're too old to masturbate (when you're too old to physically play, that's when you're too old to learn guitar).

How many hours would you say it takes before one can be professional. .... This stupid computer doesn't have the calculator accessory. What in the sam hell...