Don't worry about the age thing. I've had my son registered for various frequent flyer accounts since he was days old. Use a fake birth year if necessary. Nobody is going to call you on it.

If you live in the US or can use flights on JetBlue, sign up for their TrueBlue program and start earning badges. It is exceedingly easy to earn enough points for a free ticket with them.

Visit Bing Rewards and Bing.com daily and you'll earn about 100-200 miles/points a month. While this isn't much, it acts as activity in your accounts and keeps them current. This is handy if you are not flying for extended periods of time.

Other programs that allow you to earn free miles include e-miles, e-rewards, mycokerewards and the various dining for miles offers where you can register a debit card.