If it is 20 yrs old, it's about had it. That's about the life of a central AC unit. Check the freon pressures and temperatures going in and out. should be about 18 degrees difference as i remember. You might need a new blower motor on the air handling unit. And it contains freon 22 which isn't allowed any more. I think they have a substitute for freon 22 for the old systems but you can't buy a system that uses freon 22 any more. I had both my heat pumps/ac units replaced in 2008 with Goodman units that uses the new refrigerant. They don't advertise and you may not have ever heard of them but when I was doing my research, I found out they were the 3rd most popular units behind Carrier and Trane. I found a business that installed them for 2/3s of what everybody else wanted for a Trane or other brands. Mine have worked without any issues since 2008 which is a big change from my original Rheem units. they were almost constant trouble. Just make sure the units you buy have scroll compressors. The cheaper compressors don't last as long. My information was valid in 2008. Systems may have changed since then.
You will notice a difference in your electric bill because the new systems are much more efficient. I think my old units had a SER of 10 and the new ones are 13. I noticed a difference in the elec bill.