You can mine and play games at the same time. I do it all the time.
Just set the miner to use 1/2 the power when you are gaming.
This only works with certain games and certain algos, if he is playing some memory intensive game and mining something memory intensive then it will not work. Some lite gaming is fine with reduced hashpower however given the amount of hours that people game its just better off to disable mining completely while gaming and just re-enable it after they are done.
Will it affect my GPU/CPU lifespan? Is it necessary to run the desktop 24/7 without shutting down? Just pretty curious.
Of course it will, just like driving your car 24/7 with the pedal to the floor.
This is true however the average lifespan of a GPU is probably at least 5+ years, usually the hardware gets obsolete. The fans are the most common failure points.