I started my boys at 4 & 7. We rented for the first year, and they got tired after only a few runs each time going. We simply made everything fun for them didn't make them do anything they really didn't want to do. Honestly the hardest part was safely geting my 4 year old onto the lift each time.
The next year we did a seasonal rental for my 5 year old and bought used equipment ($70 for everything) for my then 8 year old.
This year we bought my now 10 year old his first new skis, which are 1 year old Vukyl Unlimiteds, and our now 7 year old got all of his brothers old stuff that fits him quite well. We've gotten 3 seasons out of my older sons gear and it should fit his brother again next year.
This year I've finally been able to take both boys out skiing by myself (without the wife) which is nice. She can't go as much as we boys like to, and it sucks leaving one kid home each time. My area ski hill has mainly 2 person lifts so I needed the older son to be able to ride the lifts by himself safely before I could go with both of them by myself...