My 3 1/2 year old is nearly 44 inches now. His big brother didn't get there until about age 6. My first one was always in the 10th-25th percentile in height until about age 10, and now is 50th. My younger one has always been completely off the charts (upwards of 95th percentile) in height since about 3 months.
Edited to say: My first was always ready for rides long before he was tall enough. My little one, I think, will be tall enough long before he is really ready............. I don't plan to let him ride Tof T, Space Mountain or Dinosaur even though he'll be tall enough. Last Christmas, when he was 2 1/2, he was tall enough for Barnstormer, and I wouldn't let him go as he was just a baby..........not enough head control I thought for all that bouncing around!