In the San Francisco Bay Area formula fleet (which is arguably the deepest, most competitive in the US), Steve Sylvester (at 61, I believe) consistently gets top three finishes. This is in a place with lots of wind and sometimes pretty chaotic water. Other guys there who consistently whip the young guns are Mike Zajicek (late 40's, I would guess) and Mike Percey (in his fifties and a grandfather). There are a handful of guys in their twenties who can hit top 5 finishes there, but not a whole lot.
In the Gorge fleet, the top racer is Bruce Peterson at age 44. In the overall standings for this season, the top four were all masters; fifth place was the first men's competitor (who should hit 35 in the next couple years).
You could argue that in amateur racing, age groups seem a bit like affirmative action for young guys. Experience seems to play a huge role, and while physical fitness is a highly important part of being competitive, the sort of fitness required is well within reach of people into their 30's, 40's, 50's, and (as Sylvester proves conclusively every day on the bay) 60's.