I'm 48 and my first official coaster was the "yellow" coaster at Crystal Beach in Ontario back in the 60's. I was probably about 7-8.
But....our next door neighbor had a coaster the dad had put together - (they had these back in the 60's that you could assemble in your yard) - so that may have actually been my first coaster around age 4! NOT like the little tykes plastic ones - this one probably went about 6-8 feet in the air - which is significant when you're 4!