I'm not sure when they hit the 40 inch mark, but I know my DD was well over 40 inches when we took her at 3 years 7 months (she was probably around 42 inches at this point). And my DS was over 40 this past summer at 3 years 3 months when we went back (he was around 41 inches). My little one is only a year old so time will tell. My kids tend to be on the tall side though.
Keep in mind though, that just because they hit that 40 inch mark, if they are close sometimes they can't ride because the ride's measurer is off. Also, the child may not want to ride the ride. My DS is a little daredevil and yet we took him on soarin' which I thought was a very mild ride and he hated it! He screamed the whole way through! I still can't understand why because he liked splash mountain and big thunder. So, while I can understand wanting to wait for the 40 inch mark, keep in mind that your little one may decide he's not going on the bigger rides anyway, lol!