Is there a recommended height or weight to be able to manage skis?
I agree with balanced mama -- if a ski school will accept them, then they're old enough. In the countries I know (Austria, France, Switzerland, Sweden, Norway, Italy), ski classes start around 3 years of age.
It's not really an age (or height!) limit though -- it boils down to the child's motor skills.
I've seen a four year old go racing down the slopes faster than I'd ever dare. The parents (close family friends) rented skis and boots to fit him and signed him up for a class at Whistler. In some communities, kids are on skis just about as soon as they can walk (and there is enough snow on the ground) if you get high enough on the slopes and far enough north.
As far as I am aware, it is a matter of getting the right size for the child. Of course, if they can't walk well (aim etc.) they probably shouldn't be on skis and I'd suggest signing them up for a class so they can learn the snow plow, how to fall properly and other critical "beginner's skills." If you can't get them in a class, they're probably too young (unless you yourself are a ski instructor and have all the proper safety skills etc.) Three - Four seems to be the typical age for class availability.