Other than women having a certain age limit where it becomes risky to give birth, no one is too old for anything. Challenge the conventions, I say. Too young, yes...some things, but not guitar (as long as they have enough strength to pick up one). That's in theory tho. Shredcow is right. <6 they won't pay that much attention to you anyway.
Back to the point tho...how old is old exactly? 80? If you're nimble enough to type, should be ok to pick up the guitar. Unless you aspire to be Paul Gilbert in like the next 2 years....