I think there has to be something of a height restriction. I can't remember, though, if there was some line to measure against, though. Oh, Tikiman? I do remember there is a red light/green light thing at the top letting people know when it's ok to go ahead and slide down.
But anyway, your 5 YO, unless she's extremely undersized for her age, should have no problems with restrictions. And if she's dying to ride the slide, there's no reason you shouldn't let her at this point, IMHO. In fact, I bet my DD would love it, too if she gave it a chance, but I just didn't want to go through the scene of forcing her to do it. If your 5.5 YO is wanting to do it, more power to you, and you'll have even fewer worries about keeping her time occupied than otherwise.