I'll be 30 when I start work--6 years of experience with a consulting firm really helped me, everyone with whom I interviewed commented positively about it. But there's a certain age where the biglaw sweatshops may start to question your willingness to put up with the long hours and BS of being a junior associate. What age that is, I'm not sure. It's probably more like 45 than 35. And if your prior work experience is at all comparable to work you'd be doing with the firm, I don't think it will be an issue at all.
I'll be 31 when I graduate in a few weeks. I agree with the above response, and I'll supplement by saying that small firms are immune to this "younger is better" problem. In fact, the opposite is likley to be true. In other words, being a little older probably makes you more likely to do business professionally, but less likely to throw your entire being into completion of a project for hours, days, and even weeks on end. Big firms expect the latter out of associates, and they know a very important truth: The older one gets, the less inclined he is to trade time for money.
The flip side of that truth is this: the older one gets, the more able he is to differentiate between what is truly important and what is not. This means we are all getting more efficient. Efficiency is more important when your top priority is pleasing clients than it is when your top priority is billing time. I submit that this explains the tendency of big firms to prefer young entry-level associates.