When a horse "transitions to barefoot," the situation may be more complicated than simply getting used to going without shoes. Although the transition is sometimes compared to a child becoming acclimatized to going barefoot in the summer after a winter wearing shoes, it is not just a matter of developing a harder, thicker sole to protect the bottom of the foot; there may be many more issues at play.
Often, with or without shoes, the foot is inadequate to fully support the horse, and the horse needs to grow a whole new hoof from the hairline down, something which can take many months. In some cases, one new hoof is not enough, and the process of producing a functional foot may take even longer.
Even a horse which has been barefoot for a while may still need to go through the process of developing a better foot.
It is said that a horse should be exercised on ground which is softer than, or the same as, the ground on which it is pastured. If this is not the case, then a horse in the process of transitioning to barefoot (or to a better barefoot) may need to be exercised in boots. Providing turnout which includes rough, hard ground is one of the best things one can do for a horse who is in the process of growing a better foot, or any horse for that matter.
A healthy horse can be useful from the age of five until past the age of 30 and can continue to enjoy retirement for some years after that. In the context of a 25+ year career, allowing the horse to work at less than 100% for a year or so, while the hoof regenerates, is a worthwhile investment.