I am going to apply math grad schools this year. But, the problem is now I'm 30 years old.
Do you know anyone who is 30 or over 30, but got admission from top math grad school?
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Knowing an answer to this question won't help you. What will help you is making a good application to a school you'd like to attend showing your skills and potential. No one will say, "Well, this is the greatest mathematician since Riemann, but, alas, he's 30". Age won't play any part at all in an application, not even competitively. Your suitability will be evaluated on what you have done and what can be expected of you.
But in any top school, the competition for a position is fierce and you will have a lot of other applicants that show near perfect records on most criteria. Make sure that you can stand up to the competition, not only in what you write on the application, but what you can say in an interview.
But if you don't apply, you don't get accepted. That rule is almost universal.
If there are gaps in your record, be sure you have an acceptable explanation.
I can't give you any well-known examples from the top of my head, but for postgraduate programs (and many undergraduate programs fwiw) the age of the applicant does not matter that much. Career gaps are not uncommon. Having a proper job after undergraduate program and coming back to postgraduate 10-20 years later is not uncommon.
I personally know a few academic who started and successfully completed their MSc/PhD studies when they were 60+.