In reply to steveej:
We took ours skiing their first winters at 3 months and 6 months - in 2 Valleys (Mottaret) both times.
Just had one adult ski pass and shared the childcare - we did have extra adults with us on one of the trips, but they weren't really up for looking after babies. They're quite happy with a bit of childcare now the kids are older and occupied by an iPad. Didn't use childcare - mine were still a bit unpredicatable (ie no routine) at that age, so quite hard to plan a good time to leave them. And they hadn't started nursery yet, so not used to it, and all seemed more hassle and exepense than it was worth.
At 4 months, the easiest way to keep them warm is to stick them in reasonably warm clothes, then in a sling under an oversized coat - much easier than dressing them in snowsuits, etc.
And yes, skiing from 3 or so. Last year the little one was 4 had his first lessons and was happily skiing independently. The 7 year old will soon be out-skiing me.