The established ski school answer seems to be six years, which is the age that most ski schools will start accepting children into their children's skiing programs. Many ski areas have day care programs that include skiing, and some will start children as young as four years. It all depends. We know that children develop at different rates, and that is normal. We know that at a young age children of the same age can be as much as three years apart developmentally, and that is also normal.
Developmental differences can be either cognitive, physical, or both.
So say at an age of six years, it would be normal for one child to be physically three years ahead of another child of the same age. That same child could also be three years behind in cognitive development. This would still be normal. Experience also shows that physical education seems to make a difference. Physical education teaches children how abstract concepts relate to the physical world.