Aye, it happens in MMA a lot.
Brock smashing Couture for example.
At the time of the fight, Brock was 31 and Randy 45. The result may be explained by the difference in athleticism. Also there may be diminishing returns to learning technique.
Take a look at Gymnastics and the age of it's competitors. I know the US had multiple girls under 20 and I think the girl that won the all around gold was 16.
Both in figure skating and gymnastics athletes are old by 28, so there must be some physical traits that peak at a younger age, like flexibility, which are very important to these disciplines.
But it's still strange that a 15 year old kid has all the technique required to beat a way more experienced practitioner. I'm thinking it may have something to do with the indirect nature of the competition ie. they are competing individually for a score, not head-to-head.