The plastic fuel tanks used in these boats will last the lifetime of the boat. Unless they are damaged and/or leaking fuel, they shouldn't give you any concerns because of their age. They live in the bowels of the boat and don't get exposed to the Sun's UV rays... and as such should last a very long time. Do take care if you are ever removing the fuel filler or fuel vent hoses... as the molded hose connection points could be cracked/damaged if not careful.
Cheers!