Why the earlier the better?
As a child enters adolscence, fear of the unknown intensifies. As a child, they will take right to their skis, and bomb down the hill gaining the knowledge that they can control their speed, prior to be relegated to fear that they can't. Granted, some very good skiers started later in life, but the best ones started almost from when they could walk.
I started skiing when I was 5. I learned quickly, advanced through racing, and into teaching. I can ski most anything that will hold snow. My wife learned young, and is also an expert skier. It is a fabulous family sport. When it is raining in Seattle in the winter (almost everyday), we know it is snowing 45 minutes away in the mountains. Takes the gloom off.
We have two. We skied with both of them in a baby backpack when they were under 1 year old. This was in Canada (I believe you cannot do it there now). Our son started walking on skis at 22 months. He was more interested in eating snow, but the next year, we put him in daycare at Whistler. Twice a day, they would take the kids on the Magic Carpet. They learned some independence (dressing, helmets, and all the gear), and had a blast.
Our daughter started one year older than our son. Our daughter has some issues with her body in space. Her ability to ski has helped her in all aspects of her life. Balance was thought to be an issue, but she has overcome this through skiing.
Who should teach your children to ski:
I believe whole-heartedly that a parent should not be the one to teach. Set an example, yes, teach, no. I will give my opinion, then an example. Skiing is not an easy sport, and requires pushing to make the student advance. Your child does not want to be pushed by you in something that is supposed to be fun.
Last winter, our whole family, parents, aunts, uncles, nieces, nephews, grandparents, took a ski vacation. My brother-in-law was attempting to teach his oldest daughter (8) to parallel turn. She broke down crying, and spent the entire afternoon in the lodge. I went in, and sat with her near the end of the day. I promised her that the next day, I would take her by myself, and we would ski a black diamond. She became so excited, and I followed through. I pushed her extremely hard, and she never once complained. We accomplished our goal! Was she proud...
Ski School and Peer Pressure
Unquestionably ski school is the best way to make your child learn to ski. As long as a sibling is not in the same class. They make friends, and the resulting peer pressure makes them excel.
When I was a child, I took ski lessons eveery week. Our children are only in ski school when we take two week long vacations, and as their ability has improved, we are less concerned with it.
I hope that you have found this guide helpful!