Elevators can be tricky to activate quickly; however, this task is made easier by stopping on a dime on top of the elevator platform, then pressing up or down to activate it.
As the video demonstrates, Samus will be able to activate elevators when one pixel of the ten-pixel-wide bottom of her hitbox is making contact with the elevator pad; however, if all ten pixels are not making contact with the elevator pad, there will be a 50% chance that the input performed to activate the elevator pad will fail. This is because if Samus is touching two tiles at once, the game's Collision Oscillator will alternate which tile to detect her as being in contact with on each frame.
As a result it's common for runners to ledge grab on to an up elevator pad (since up elevators are always on raised platforms) and spam up until they succeed in starting the elevator. Meanwhile, for down elevators runners are much more careful in general to make sure they're fully on the elevator pad before pressing down. A crouch or morph ball costs a significant number of frames to get out of so you can try again.