My job offers a 401k plan, and once I have been there for 6 months I will be able to take part in it. I've been there 5 months now. I'll be 20 at the end of this month and will be starting my 401k as SOON as I'm able to.
I don't think you're ever too young for one. If you're lucky enough to have a job that offers it, why not take advantage?!