My dad has had a cartlidge op on each knee since the age of about 50ish. Reality is that having run mostly road since the age of about 16 his knees are shot. He's very mobile, but hasn't run since his mid 50's.

He asked one of his consultants about cartlidge replacements, but was told that they're working on it, but it's a long way off being available. He's 74 now, so I guess things may have moved on in the intervening 20 years.