Defrag should be done every 1-5 month depends how much work you are working on it. It will not do any harm to the computer so you could do whenever you feel like it.

But please know Defraging will not increase the speed of your computer, it will only decrease the amount of speed is being used.
You mentioned your computer is 8 years old then you should be considering upgrade your computer for a better one or buy a new one.
Upgrading ram is cheap and is the best way to speed up your computer.

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