What has already been said is pretty much on the mark, so I'll just add some more anecdotal info.
I started my daughter skiing when she was just over 3 y.o.. Within a couple of days, she had absolutely no problem with upper level greens, never complained, etc.. My real problem with her was that she never knew when to quit and wanted to progress to blues and blacks in that first year. She certainly could have safely descended such trails (if empty, good snow, etc.) at 3 y.o., but it was waaay before her technique was ready. I didn't want her to become another power-wedge guided missile flying down the slopes in a straight line with zero control, so I didn't let her get up on serious blues and blacks for another couple of seasons.
OTOH, kids are wildly different in their athletic ability, coordination, maturity, desire to ski, etc.. When our kids' program is short of instructors, I sometimes am pressed into service and have had to deal with 7 and 8 y.o.'s that should clearly not be in ski school yet.
At the other extreme, my best child student last year was a private with a little Chinese girl who had just turned 4 y.o. She was an ice skater, and I had her doing skating step turns down the bunny slope, hand-in-hand with me, giggling all the way, and doing so within a half hour of her first putting on skis. Heck, I couldn't do that with 95% of adult students.
The bottom line is that the most appropriate age to start a child on skis varies all over the place.
Tom / PM
PS - BobB would have been very proud of that little Chinese girl. Right from the start, I had her saying (and thinking), "let's GO over there ... lets GO over here next and check out that snow gun ...what's that laying in the snow over there, I'll 'race' you over there to find out, ... etc.". Because "over there" always involved completing a turn and sometimes even going back up hill a bit, never once did it look like speed control was an issue for her, even on that hardpacked, semi-icy night.
The strange thing was that her parents positioned themselves at the bottom of the bunny slope and watched the entire lesson. When I spoke to them afterwards, they just couldn't seem to understand that the progress she made was extraordinary. In fact, they then wanted to turn her over to her 8 or 9 y.o. brother who was power-wedging all over the mountain - arghhh.