Both our kids went out on cross-country style skis and soft boots at 2 -3 years old to play and walk around. Real downhill equipment came at 3-4 years - later for our daughter, earlier for our son.
At 2 1/2 our son wanted to ride the chair because he was facinated with machines. But within a few days he announced in line
"I am learning to keep my balance. I am learning to ski!", to the disgust of the adult beginners. I didn't even
KNOW he knew what balance was. Our daughter at this age mainly puttered around. She really connected with skiing at about 5 years old.
We had skis with Mickey and Donald, (or was it Minnie?) on the tips to identify right from left. Our daughter, then 5 (?), would go backwards facing her little brother calling out "Mickey, Minnnie, Mickey, Minnie" and our son, with his hands on his knees, would dip his shoulder, going into perfect turns. This got our daughter moving on her skis, engaged and loose....and by the next season our son was doing wide stance parallel turns down about anything. We kept him on a leash system when he was 3 -- until he could clearly do hockey stops to shut down the speed. I kept envisioning a pylon in his future... I had to ask the ski school teachers to not force him to do a wedge. Once they saw he really did have control, they usually let him be.
Our daughter was more socially minded with her skiing. She wanted peers - that is what got her engaged. Kindergarten and 1st grade gave her friends to ski with, via our shared ski lease and a school ski trip. She really bloomed, and then began to enjoy it all for its own sake. Or at least that's what it seemed like at the time.
Our son by 5 was intrigued with snow boards. So we told him when he could do linked parallel turns down black diamonds, then he could try boarding. The spring he was 6, we all took a boarding lesson....and he has never been without his skis since. :D He hated the lack of control on the board, when he had such mastery on his skis.
Ski school with these two was a challenge. Our son had the skills our daughter had, but being nearly 3 years younger, not the mass and hence not the speed. Yet far more assertiveness. (our daughter just recently observed how many second children she knows who are the more aggessive skiers, regardless of gender. )
In group lessons in these early years our son would lag behind, just because he was half the size of the others of his skill level. Even worse, the teachers would pull him out of his ability group and put him with his age group ie. basic beginners with no poles.
Once time when I saw this I skated over to the instructor, who graciously invited me to ride the chair with him. When I pointed out the parallel tracks in the powder under the chair as my son's, he got the point.
We finally opted for private lessons, though expensive. It kept the focus on their skills, not their ages. We preferred pro instructors, and tried not to "teach" during family skiing. We held family skiing as a treat the kids earned by going to classes. We kept them in structured learning until they were truly at an advanced / expert level.
They hated it then, and are glad for it now.
At 3-4 years old, if they did 5 runs it was a full day. I've schelpped
a lot of toys and cozy blankets for naps through parking lots of slush and cafeterias of ground up chips.
In retrospect, we are sorry we did not involve them in one of the Mighty Mite programs -- 4 to 8 year olds. They went onto team programs later -- middle school for our son, high school for our daughter -- in part to meet peers to ski with. Through ski school and privates they got the skills, but missed out on the social community.
So what age is best? It is a chemistry of temperment and motivation, size and durability in the cold and in a group setting.
Jen