and buy keys that look like this
there are simpler designs that your fingers will find dificult to turn if stiff. There are big ones that are very good but may not fit.
Buy several because they get lost.
One in the kitchen drawer, one in the bathroom, one in each toolbox, one with a piece of string through the hole, hund on a hook next to the boiler.
Have a sponge ready to hold over the valve because the water may be black with iron oxide that leaves a permanent stain on clothes, carpets and paintwork.
but
Why do you want to bleed them?
A radiator that needs bleeding will be hot at the bottom, hot in the middle, hot at the sides, but cold at the top.