If someone doesn't start racing until they're 40, it's completely possible to continue to improve into their 50s (or later). Assuming that they start training early in life, they can keep getting faster into thier 30s. Generally, you won't see short course pros much past their early to mid 30s. Of course, that doesn't mean that there aren't plenty of fast 50 year olds out there racing, kicking some 25-year-old butt....

Well, Joe is a huge outlier that really skews the curve!
For years it's been common theory in the running community that it takes ten years for someone to reach peak. I think this is borne out in the ages of runners and triathletes and cyclists who reach the top...they are usually in their 30's. Gator is right that even if you start in your 40's, with good, consistant training you'll reach peak in about ten years.
I think longer distances are dominated by older athletes because older folks tend to have a better mental game...more patient, more willing to roll with the punches...that kind of thing. Not to mention more experience generally.
The ten year rule is quite fluid. If training isn't consistant or you lose a lot of time to injury it will have a big effect.
Remember...everyone has a performance cap. There will be a point when it takes huge effort to improve even by a few minutes.
There are limits your body will come up against that you just won't be able to pass....even someone like Joe.

"older folks tend to have a better mental game...more patient, more willing to roll with the punches.."

I am finding myself agreeing with that statement more and more as I am growing old myself. It's a great sight to see the 50+ AG at races looking strong and fit and kicking ass. It gives me a completely new outlook on my older years. I don't have to grow old tubby and stuck in an armchair. I love that.