Babies will happily crawl off of the edge of whatever they're on. I'm not sure if it's because they aren't afraid, or if it's because they're so used to being carried that they don't grasp the concept of gravity, or both.

My toddler recently went out on our roof to retrieve a football. I expected her to be a bit nervous, but she walked right up to the edge, no fear apparent at all. I had to desperately shove my instinct to yell for her down so I didn't scare her and distract her.

Here's an experiment showing 27 of 36 crawling-age infants had no apparent fear of heights: https://en.wikipedia.org/wiki/Visual_cliff

IIRC very small kids are also developing depth perception, they aren't born with it.