Why do drivers tend to lose it at around their mid 30's? Is it just a physical thing or is there more to it?
Personally, I think that what makes a driver fast in their own particular timeframe is only really useful for about 10 years, and then they either have to adapt or fall away as the new guys come in who are completely adapted to the new paradigm...
It's more obvious in bikes I find... look at Marquez, he is someone who probably isn't that much better in relative terms than Rossi was in his peak, but he's hit the paradigm of what makes the fastest the fastest at exactly the right time. Same for Hamilton in 2007... his style was perfect for that time and he jumped right in and was a point shy of the title.
Damon Hill was one of the top drivers of the 90's but was 33 before he even got into the Williams, but that's because his timeframe had shifted due to his later start in racing, and despite his age he was in the same development curve as his younger peers. However, his age meant that when the paradigm shifted again (in 98/99 when the narrow cars and grooved tyres came in) he was unable to adapt and found himself quickly irrelevant.
The new kids aren't any better in real terms, it's just that they're better for what makes a driver now. Even a genius like Alonso in his early 30's is living essentially on borrowed time, and his peers like Button and especially Raikkonen are now starting to show signs not of ageing, but of falling behind the curve.