When I started looking for information about PhD programs in the US, I soon noticed that the issue of age of PhD applicants in the US is hotly discussed over the internet. That really surprised me.
I was 43-years old when I applied, turned 44 recently. And when I tell people in Brazil that I'm going to do a PhD, several of them say that it is good, since I'm still very young. It is not really strange to see people of my age doing a PhD around here. But in the US I'm considered an older applicant.
And I think that one of the reasons is that the typical road to a PhD in Brazil is much longer than in the US. There are fewer universities, it's hard to be accepted by good undergrad schools and often takes years of preparation. After that, it's almost impossible to jump to a PhD. You must first have a Master's degree. Which is even harder, and usually also takes a lot of time. So, when you get to the point of applying to a PhD, a Brazilian applicant is on average older than a US applicant.
No matter the reason. According to discussions I've seen in the internet, no school will admit that age is an issue, but that is indeed the case for some schools and age may be help against me. Not many schools, of course, but it's hard to know.
My personal opinion is that you shouldn't let your age stop you from applying to a PhD in the US, if that's really the right thing for you. Ok, maybe you don't have the same level of energy when compared to younger people, but probably you learned a few things during those years which will compensate, like better time management and capacity to deal with pressure and failure. Each person is different, you should evaluate that yourself.
And don't fall into the trap of thinking that it's just some stupid ageism. They may have valid reasons to be concerned, and you must show them somehow that they do not have reason to worry. It does make sense to be worried about the academic sharpness of someone who has been away from school for many years, for example.
So, instead of worrying and complaining about the age issue, do your best to develop the strongest application you can. I think that's valid for any kind of prejudice and discrimination anyone may face when applying to a PhD.