I'm 25, and currently have three partners. *shrugs* I don't know if age has anything at all to do with being poly...I think it is more a matter of maturity and knowing what you want and need from your relationships.
I think (generally speaking), when a person is younger and more inexperienced they are more likely to have multiple partners and be dishonest about it. And (generally) as people get older and find out what they want and need, they are more likely to have open, honest poly-type situations. Where there is more than one partner, but everyone is privy to the situation.