I would say most of the people here are agnostics not atheists, hence the name of the site. Maybe not that many people know about this site, so the ages may skew higher. But I think the majority of atheists and agnostics are younger than older.

Based not only on my own experience but on other discussion on the Internet I believe that the assumption that most atheists are young is because the actual atheist movement is young. Atheist was considered a bad word until atheist began fighting back. Hardcore religious people still consider it a bad word but the atheist community is now pointing out to the world that we are your neighbors, your parents, children, friends etc. Somewhat similar to the gay movements of the 70s and 80s.

So what has happened is we older atheists realize we are not alone and we look for community support. Maybe younger atheists have that support already in their peer group.