Our foals do not get worked on the longe line until they are 2 years old, and even then it's rare for us to longe one--what it amounts to, as 2 year olds they learn how to longe, they do not get conditioned that way.
Our yearlings and 2 year olds get conditioned by working free in a big corral (as in 60' x 120' )--there are two of us, and we stand one towards each end of the corral, and keep the horse moving--gives them room to move without putting stress on their developing legs.
We start halter breaking our foals when they are a month old, and the only "work" they get as weanlings is going for walks and learning to set up & pose for showing.