'Neoteny'?

Getting off topic here but... "neoteny"...I've heard of "paedomorphic", but this is an interesting term I've run into for the first time ever. So that must have been the more precise word I've been trying to find for years to describe the physical nature of pixies. They are the homonid analogy of a toy dog. It is their inherently soft, diminutive, ectomorphic and effeminate physical makeup that largely contributes to, for example, a typical 10 or 11 year old pixie almost routinely looking more like the age of the usual 5 or 6 year old Germanic ... so is this the precise word I've been trying to find for so long? Just double checking here. If so, it certainly will be a very handy word for me to add the the vocab:)

Also, what then is the ANTONYM of neoteny? I have also seen converse situations of children or youth who look a LOT older in certain kinds of ways than what they actually really are. I saw a girl just the other day who was 10 but had that certain kind of plumpish "auntie Edna at age 55" look (if that's an apt way to help those here conjure up the image in their heads). Anyone ever come across girls like that too?