Harps can last a heck of a lot longer than 20 years. I know of harps that are around 100 years old and play just fine.
Harps do have a limited life span. The stress on them means that they will only last so long, unlike some other instruments that just improve with the centuries. Of course you have to take decent care of it in a proper environment.
I don’t know what the average life span of a harp is, but I know of many, many harps out there well over 20 years old. Heck, my current harp is almost that old now.