I knocks years off my age. I am wearing it as I type.
I have seen a couple of older teenagers and 20somethings in them on the train or in the street but for me they are like Ugg boots, something you only wear in the privacy of your own home.
I used to wear one piece overalls at work, the main disadvantage was that you sat stark bollock naked on the toilet. I would imagine a onesie has the same problem which, as it is intended to keep you warm, seems to me an insoluble problem.