Too young to be poly...too young to be in love...too young to be wise... There are a lot of things that people are supposed to be too young for. I don't believe most of them. Too young to understand all of the consequences of your actions before acting? That one I can agree with only because when young, as Eugene pointed out, everything seems more urgent so one is less likely to take the time to figure out all (or even as many as possible) of the consequences. Then again, a lot of older people don't do that either.
One is never to young to love, so one can never be too young to be poly, just as one is never too old to love, so one can never be too old to be poly either.