I have a computer which will not used, however, it is about 4-5 years old already,
is it worth to use it to mine BTC?
If yes, how much will I expect to get in 1 month
It is worth because you contribute to the bitcoin network, as a charity.
You wont be earning anything because its already past your difficulty, but as a charity you can do it

I wouldn't say that this is accurate. You are only contributing to the network if you are solving blocks(for solomining) or submitting shares (for pooled mining). Otherwise, if your computer is not powerful enough to submit a share, you would be just wasting your own resources.
Even if your computer is powerful enough, you should buy a USB ASIC miner and a raspberry pi. Those are much more powerful while consuming far less electricity when mining.