One of the most amazing sights I've seen on skis was a guy teaching his 7 MONTH old to ski. Our group spotted this guy on the hill and went to talk to him because it was pretty obvious the tiny thing was way too small to be on the slopes. To our surprise, this little girl was having a ball (I mean an absolute ear to ear grin) and was actually turning the skis while coming down the hill. Mind you, the little girl was not walking yet and had (Dad guessed) 20% of her weight supported. He was using a T bar device that is a kid-ski product from Applerise
We saw this gentleman on another day with his 2 and 4 YO who had done the same program that the 7 month old was doing. The 2YO was skiing unassisted and the 4yo was ripping. This is not for everyone. I would have said it was impossible had I not seen it in person.
At our mountain, we have group programs that start at 4 years of age. We occaisonally will let advanced 3 yo sneak in (through parents lying), but usually require 3yo to take privates. When starting extreme youngsters it is important to go at their pace. A 1/2 hour - 45 minutes may be all they can handle at one time or even for the whole day. Quit BEFORE they get tired and frustrated.
My experience is that most (not all) very young kids are speed freaks. If you can get them (balanced and) over 10 mph - they are hooked. It also helps a lot if they sense you are in control by being right there if they need you, but not necessarily in physical "controlling" contact. They need to be doing it, but they need you instantaneously if fear crops up.