As horses get older, their ability to absorb nutrients declines. Exactly "when" that happens to a degree it warrants an increase
in vitamins/mineral/caloric intake varies w each individual horse and usually becomes apparent with all things held equal (diet, environment, etc.,). Nutrients, in turn, help ensure they get the biggest "bang" out of all their calories. So, senior feed "delivers" on these fronts and is convenient. I have fed senior feed to young horses when I was trying to boost their weight and "ensure" their dietary needs were met. It can't hurt, but "when" a senior diet plan is necessary for a given senior isn't necessarily determined by their birth date.