Yeah, you'll notice a lot of older folks squatting when they rest (farmers and the like). I think if you are used to it it becomes a natural position for you.
As for seiza, I don't know how you get used to it, really... I played Japanese music for years and you have to sit in seiza for hours at a time, and it was always so painful. Even the older ladies complained from time to time. It's actually a great way to sit as far as your back goes, and helps you have perfect posture for singing, playing, etc., but it just murders your legs.