I actually agree with Leia on this.
Narrow behind is hard to do anything about, and what you do accomplish will be cosmetic, but if I had a dollar for every animal I have bought with a narrow front that has broadened, naturally, with exercise and discipline, I should be rich.
And it is as permanent as the muscles tone, so yes, it is permanent.
Personally I like free running in a round pen, equal times on both sides, with ground poles.
Once they have settled down and stopped jumping the poles, they have to reach down and stretch their necks and fronts in order to navigate the poles.
This is natural and unforced, and I have found it is gentle enough to use with quite young animals, although obviously you do have to be very careful with anything under two/three years old.
With a five year old I would start slow and short and build up slowly over a week or so to anything up to half an hour, so long as the horse was going steady.
It is very important to find out which side the horse favours (left or right, horses are "handed") and make very sure that you do not allow it to go longer on one side than the other.