Senior horses need more protein than "regular" horses because of their decreased capabilities to absorb it. This means a higher protein grain, or a hay higher in protein than regular grass hay. They will absolutely need their teeth floated, usually 1-2 times per year (or more depending on the horse) - and at somepoint, all of their teeth will fall out = mashed grain.
They will still need their feet trimmed, innoculations, etc.
They don't quit being horses just 'cause they're old
That'd be like throwing your granny out and not giving her any attention