I've racked my brains and realised mine is being driven past Gwernvale
http://www.themodernantiquarian.com/site/530
as little child every year on the way to visit relations in Wales. I don't remember what I thought it was, but I know I used to look out for it as a landmark, and I knew it was something weird and old. Otherwise, the obvious memory is Stonehenge at age about 11. But I know I was into archaeology before that (maybe just R*man things. there was more of that about in our neck of the woods).