It's a nice axiom and quite respectful but rarely true. Perhaps it holds true when comparing the untrained to the trained but that's it. It's why we don't see 60 year old boxers and other fighters schooling the young bucks, because age and ability are not exponential. Strength and power are not to be underestimated and it takes colossal skill to overcome size and strength, and when a guy is big and strong as well as skilled, age and experience are not absolutely better. However, age and experience coupled with strength and size can be quite formidable, that's why we've seen quite a few fighters compete into ages older than other competitive sports.
That's one problem with martial arts, if you're into it long enough eventually you'll be an old guy still training martial arts. The funny thing is all the old guys I meet in martial arts all talk about how hard they trained back in the day and how much better they used to be, along with the martial arts scene as a whole. Some I believe, some I definitely don't believe. It's not hard to simply show up to a class and next thing you know 30 years have gone by, you may even have done a competition or two, but that doesn't mean you were ever that serious. This is just an observation. Age does not guarantee skill.