I tend not to hire anyone who no longer has control over their bodily functions.

If you strip away stereotypes about lacking energy and dynamism and so on (which are generally as worthless as any other stereotype), the only genuine factor I can think of is how close is the person to retirement and how long before they leave you.

Given that it's not uncommon for a programmer of any age to move on after 2 - 3 years, this is basically a non-issue so long as the person is smart, knowledgeable and hard working I wouldn't consider it.

Oh, and in the UK at least as an employer I should probably mention that considering someone's age as a reason for hiring or not is illegal - though obviously very hard to prove that that was the reason.