If your horse is tired because it's old that's ok, just give it electrolites!
Sure.. give it electrolites if it needs them... but if your horse is old and tired... consider doing less. Maybe even nothing.
I don't think horses need retired by a certain age, or that once they've been retired they should never be brought back into work if things change... but know your horse.
I know my 28yro TB isn't up for the same riding she used to be... Trailrides are harder for her now so we have to take it easy but they tend to bring her back to life a bit and she eats better and jut does better in general when she's being ridden so for now I'll still push her a bit if she feels to sleepy to go on a trailride, as long as there's no sign of lameness or pain causing her disinterest. She usually changes her mind part way through the trail anyway.
I do look at it the same as I would for an older person, they don't need to be out working or jogging but if they can continue to stay active as much as possible for as long as possible it does help their health and overall well being. Even if they don't enjoy the actual exercise, they enjoy the benefits.