A lot of people do not think about this until they have to. When they do, they are more concerned about buying their new computer than about disposing of the old one. Some are kept lying around somewhere in the house or office for ages. Some are thrown in a skip or wheelie-bin. Some are taken to the tip.
Many of them probably do not need to be taken out of use. They can be cleaned up and given a new lease of life at a reasonable cost. The thing that slows them down and makes them seem “past it” is the amount of unwanted and unused software and out-of-date data we all allow to build up on our computers. A good purge can work wonders!
The thing to be concerned about is the security of your data. Hackers can usually recover data from discarded computers. Even ones you thought were not working.
One option is to physically destroy the machine, for instance by smashing it up with a hammer.
Another way is to find a trustworthy expert to wipe the data and recycle the machine. I know one or two who are properly approved and licenced. They sell the newly secure and reconstituted computers. I was surprised to learn there is a market for them. That way your data is safe and you are contributing to the environment by recycling. Not bad!