It's far too premature to say "yes, you need to replace a fuel tank on a 25 year old boat" without knowing more details of the boat and how it was used and stored. Was it stored under cover? How likely is it that the foam surrounding the fuel tank is wet or compromised? Can you see the top of the tank? How does it look? Have the hoses been replaced yet? Do you smell any gasoline in the bilge?
Again ..... way to early to say that a fuel tank needs to be replaced. As a case in point, I had a 1986 Parker. Fuel tank was fine when I sold it in 2011. I stay in regular contact with the current owner. We spoke several months ago and the fuel system is still fine. Every situation is unique and must be evaluated individually.