too old to spin?

At what point in our life does spinning start to make us feel sick? I have watched in amazement at my children spinning for twenty minutes straight and it just blows me away. Sure, they still get dizzy and their eyes are wiggin' out when they finally stop, but they never get the nausea that we get as adults. I remember being able to spin like that when I was younger. So when did it change?

I remember throwing up at a carnival after riding the "Scrambler" four times in a row. I think I was about twelve years old at the time. Perhaps that was the onset of my centrifugally challenged existence. But what causes it?

I understand that our sense of balance has something to do with our ears. Perhaps our "new ears" during our youth haven't established that same sense of balance (or the ability to get sick when it isn't detected). It's really difficult to say. None-the-less, I still enjoy riding roller coasters and hope to someday take my kids on a vacation near Ohio, where they seem to have a plethora of great amusement parks with lots of great coasters. We've never even been on a twin track racing coaster yet, although they've been around for centuries.