Lots of electonic hardware gets problems with the electrolitic capacitors

My first question is how old capacitors usualy affect the capacitance of that electrolitic capacitor?

Second how will affect a bad electrolitic capacitor on the input of the PSU? And how will it affect if the bad capacitor is on the output?

Note: i have measured the voltage several times and it is usualy correct but the electronic equipment only starts after replacing the capacitors