That is really interesting-because I've never encountered that where I live (not diss-ing you AT ALL).
I don't think that there is an age that one could be defined "poly".
I do think that because society isn't "pro-poly" a large majority of the people living some sort of mono life are going to come up with any excuse possible to keep from labeling you poly.
It's like a parent who says their child is"just a normal little boy, that's why he's so distracted and hyper" when in fact he's ADHD. They don't like the connotation of the label.
It has NOTHING to do with the child in particular-everything to do with the parent's fear.
In the case of mono/poly I think it's a case of fear from poly's that you might REALLY BE only testing the waters (which often DOES occur in youth-but not always) and a fear from mono's that labeling you poly will mean approving of it and that might make them less than they think they are as mono's.
"Love As Thou Wilt"