In my denomination (http://www.opc.org), an elder is a servant of the congregation, teaching, administering the Lord's Supper, providing counseling to members, and generally overseeing the work of the church, among other things. Elders work alongside pastors. Here's a good summary: http://en.wikipedia.org/wiki/Presbyterian_polity#The_Elder
Elder has different meanings in different denominations, and isn't unique to Protestants (it is, however, mostly Christian). Specifically, in the Mormon church, it's quite different.
While most elders tend to be older men, there's no minimum age requirement. I am young for an elder but there are pastors in my denomination in their early 30's. It's unusual but not unheard of.
