I think one knows at an early age if they are poly...although often the tools and as LR said the "labels" aren't known. I think we all try when we are younger to fit in the mold of what is accepted but eventually (like in your 30s and 40s) break free from the need to be part of the pack. I know over the last couple of years I have been trying to find peace with who I am and what I really want in life. In the past I have given up on a lover in order to explore a new lover's world because usually the previous lover wasn't okay with allowing me to develop another relationship concurrently, so I thought there was something wrong with me... because one person didn't make me completely happy and although I still loved former, I also loved latter and traditionally speaking this must mean you haven't found The One. So a strange and fast-moving cycling of people into and out of your life (although I have remained good friends with most of my formers) happens. So in your 20s/early 30s you decide family is the next step (well for women this is kind of biologically forced on us) and so you must find the most compatible of souls who you think will be a good parent to your children. The next decade or so you live somewhat blissfully in the business of raising kids together. Then once the kids are more independent and you have more Me Time you both start to rethink life and direction and self-happiness. With all the self discovery as a focus that also means any kind of suppressed parts of yourself have a chance to resurface often for the first time as open and public. If you are poly that means discovering new relationships. If you are mono in a relationship with a poly I guess that means you find your own inner-strength and ability to love unconditionally. Either way the older you get the more open you become with who you are. But really, if your are a confident introspective youth then you know from the beginning. Just one girl's opinion!