People recycle usernames, passwords, and social media profile pictures.Grab their profile pic from their Facebook or Twitter account and plug it into a reverse image lookup such as Tin Eye.
And it should, because Google is a powerful tool (especially ).
But if you don't know anything particularly identifying about the person you're looking for (such as their email address), it's better to skip the fancy search hacks and go straight to plugging in keywords.
Facebook is the most popular social network, and it has the most robust search engine, so you should probably start there.
Facebook's People Search lets you search for people by filling in one or more search boxes: Name, hometown, current city, high school, mutual friend, college or university, employer, and graduate school.
On their Linked In profile, they've probably listed their undergraduate degree and when they graduated from college, which means you can work backward to figure out approximately how old they are.