As far as I know their reason for the age limitation of 25 is the fact that they noticed there was a higher failure rate between elderly people. Why they choose 25 as their limit... I don't know.
When I called Eurocontrol to receive feedback on my previous test I asked if there would be any difference in the tests for me because I'm just within the age bracket. They told me that the phase 2 test which I'll have to retake will be the same but the interview will be harder.....