I just entered a MS in Computer Science in a mid tier state school. I'm interested in taking things to the next level, but I just turned 30. By the time I do the math and everything, I'll probably be 34 before applying. Is that too old? Does that essentially eliminate me from top programs?
Age is simply not a factor in PhD admissions in computer science, at least in North America. As Zenon says, age discrimination is illegal here.
However, being out from school for a significant period of time (say, more than five years) is a potential hurdle for PhD admissions, at least in my department. We typically recommend that those students pursue a MS degree first, to refresh their academic background. But since you're in an MS program now, that shouldn't be a issue for you.
Apart from admission (which normally should not be related to age), there is something that's related to your question. I'm going to generalize and say that the older one is, the more that person will have financial restraints tied to lifestyle. One tends to be married, to have children, to have a mortgage, to have debts, etc. You may not agree with this generalization, but many students seeking PhDs at my university fall into this category.
In such cases, the problem is more about funding requirements related to lifestyle. Funding opportunities are always limited (regardless of one's age), but it might be harder for someone who has financial obligations to manage with the limited funding for a PhD. I know of several cases of PhD candidates who never started, or (worse) didn't finish because the funding was too limited for them as spouses, parents, homeowners, etc.
Age is not a PhD admission facor in the continental Europe, too.
The selection is based on titles, publications, curriculum vitae and generally on what you've done.
I just want to add that, being a current student in a top 10 CS PhD program, I don't feel that age is a factor even in 'day-to-day' interactions. There are several students in our department who worked in industry or had a career for 10+ years before they decided they wanted to come back to grad school. We all get along fine, those of us in our 20's, 30's even 40s. I'm sure that has a lot to do with training to be scientists. We make a conscious effort to be as objective as possible, and this is one instance where that kind of thinking benefits everybody.
In my field, which is not computer science, we get a number of clinicians who are a few years away from retirement applying for PhDs. NIH institutional training grants track students for a number of years post graduation so accepting people who will retire soon after graduation is bad. Of course these older applicants are never rejected based on age.
The problem arises more for applicants that are nearer 50 than 30 years old.
In Turkey, age is a very important decisive factor in all types of graduate school admissions and other academic pursuits. Usually, there is an age limit of less than 27, to be eligible for applying scholarships or other types of funding under the title of a student.
Also, Turkish professors are not willing to advise older students, because the structure of the Turkish society requires older people to get married and grow their kids, instead of being a student. As a consequence of this, woman in academia are usually positively discriminated resulting in lower teaching loads and research expectations to be satisfied. This is due to the fact that they could serve as good wives for their husbands, and grow more kids. Almost all woman academics are in this business (in academia), in order to benefit from opportunities to become better housewives and mothers.