Yo, Becky! My son is 6 years old and he was 1/8" shy of hitting the 48" mark last week. Hitting the 48" mark would have entitled him to be tall enough to ride all the coasters at Walt Disney World. In fact, the only attraction that has a height requirement higher than 48" is the Tomorrowland Speedway, which has a 52" height requirement to ride alone.
As it was, at 47 7/8", my son was able to experience everything but four attractions: Rock n' Roller Coaster, Primeval Whirl, Sum of All Fears and riding alone on the Tomorrowland Speedway (he did drive the car with my wife riding shotgun.)
Believe it or not (
you can trust me - I'm not a doctor, but I play one on TV...) the vast majority of attractions at Disney World don't have any height requirement at all. The attractions that do have height requirements generally hover around the 40"-44" mark, including coasters such as Space Mountain, Big Thunder Mountain and Expedition Everest. Be sure to check out the
full list of attractions with height requirements as you plan ahead.