How can I repair my carpet that is about 20 y.o?
Is it ok to repair it instead of throwing it out

It depends. Can I see a pic?

I agree with harmargedon about the fact that everything depends on many conditions and factors in this matter. And it would really be better if you attached a photo. So you can sensibly assess the state of the carpet and understand what actions need to be taken. Of course, it would be nice if it could be restored. Because it saves money.
On the other hand, since you have already had it for 20 years, it means (I draw the conclusion myself) that it is dear to you. That is why you are interested in the issue of carpet restoration. Now you can find a huge number of services that provide such a service. For example, I can advise you https://procarpetrepairs.com.au/. You should consult with these guys to get high-quality help.