Frankly, it's a problem.
Age discrimination is real, and I believe it starts as low as 30...depending on the agenda and prejudices of the employer.
My take is that employers fall into two basic categories to some degree or another.
1. The Exploiter - they want someone gullible enough to live for their career at the expense of anything else that should be more important. For these types, older people run the risk of "experience" teaching them that some things come before career. They want an older person to prove their blind dedication to job before all other things.
2. He Who Has High Expectations - they want someone who has proven themselves, and the older you are, the more "accomplishment" they expect to see. Older people who have held low-end jobs or have nothing but education to show off are perceived as incapable or untested and simply not desirable choices.
At the age of 30, you can get it from both angles, and getting older doesn't help...especially in a tough job market. A young person might be hampered by inexperience, but most employers account for that when they evaluate a candidate. An older person with an unimpressive resume really is at a disadvantage.
They are not supposed to ask your age....but many places will have a spot for you to declare when you attended school. This is really inappropriate, but you are correct....if they know when you went to school (or graduated) they can reverse-calculate when you were likely born. I finished graduate school in 2001. Presuming the average college graduate is 25, add 3, that's 28 (I actually was 33). It's now 2009. So, they know I'm at least 33 or older just from when I finished graduate school.