I'm not an expert in these matters but I have some experience with laptops, and what I've noticed is they tend to "die" because of the dilation produced by sudden changes in the machine's temperature wich frequently makes the GPU chip to disconnect from the motherboard.
I would recommend to keep the computer's temperature as constant and low as possible, doesn't matter if it's on or off as much as if it's temperature is as constant and as low as possible. Avoid playing video games in high video settings and avoid high demanding tasks (maybe video editing, recording).
Suddenly powering off you computer could create memory errors which may slow down your performance, they may even force you to reinstall parts of the software (sometimes you'll have to "repair" the operating sistem), but I'm not sure if it's bad for the hardware other than a sudden change in its temperature (but it is certainly not good for it either).
So, answering your questions:
and which hardware of a laptop computer is easy to aging?
I would say gpu and cpu are what's most likely going to force you to buy a new laptop
If I sometimes force to power it off without correctly exiting the operating system, how bad will it "hurt" the laptop?
Software errors which may make your laptops life a bit harder.
is it necessary to turn it off and give it a rest after having used for some hours?
Not really as long as it's not working at super high temperatures for hours.
what are the best habits of using a laptop computer so as to be able to use it as long as possible
Clean it once a year, these machines tend to get full of dust specially on the fan. Don't make it work at high temperatures for extended periods of time. And try not to keep the battery constantly plugged, that will reduce the battery's life.
Hope this helped
