IANAL -

But AFAIK, hiring the older worker on the basis of age would never be illegal. Discriminating against an older candidate is illegal in certain circumstances, but not always.

http://www.eeoc.gov/facts/age.html

In practice, such discrimination appears to be commonplace and virtually unpunished given the burden on the candidate to prove that discrimination actually occurred.

Considering that there were 22,000 Federal age discrimination claims in 2009 [http://www.eeoc.gov/eeoc/statistics/enforcement/charges.cfm] and that 61% of all claims were considered to have no reasonable cause and 17% were closed administratively [http://www.eeoc.gov/eeoc/statistics/enforcement/all.cfm] one could napkin estimate that about 5000 people had some success in claiming age discrimination. I'd be willing to bet that most of those successes were against an employer and not related to the hiring process.

Not that any of that makes it right.

Plus, the younger employee will probably be cheaper to insure, and may not have kids (although at 55 someone may not either, and if so would be less likely to gain them). All of which is highly illegal and lame, but may be a factor at some jobs.