I am not sure it is true that mathematical intelligence peaks early, people like to say it though and I'd be interested if there is evidence on it.

Here are a lot of examples of good contributions by older mathematicians.

https://math.stackexchange.com/questions/1059235/great-contr...

I think one factor is that young people tend to be hungry to achieve something and also aren't burdened with family or admin which means they can work harder.