Indeed, you need to consider CAPS early because it requires altitude, in a plane with a problem you are presumably losing altitude, maybe rapidly, so it won't be long before you can't use CAPS. By then you need to have either decided to use it, or picked some other course of action so that you don't need it.

Unlike the rocket ejector seat in a fighter jet, CAPS cannot save you very close to the ground. The ejector seat in a modern fighter is rated for zero/zero ie you could pull the handle from a plane that's just sat on the ground motionless, it would still eject you upward and you'd probably survive. CAPS is intended to be used at two thousand feet AGL (ie above the ground)