I am a PhD candidate in Comp Sc and will soon touch 30. And I don't think I've grown slower during the years. My intuition has probably never been "better" and I have nurtured the humility to understand that I am far from perfect, even in my opinions and thoughts. But the pursuit is to be perfect and I know that I can do a great job.
My question to you is: Are you sure that the "slowness" as you've got "older" is really true or is it something you've learnt from the popular culture? It is easy to get biased, very difficult to be true. This is, in particular true about the scientific theories of the effects of age on mathematical ability. Just take note of how many papers are published each year contradicting the earlier theories.
From my personal experience I can say that when you enter the graduate school late, the expectations of the people around you are different -- most likely you'll have considerably more responsibilities like family, planning for the future, etc. So you surely need a stronger will power and time management skills than the under 25. This is an opportunity.