It's never too late to try to slow down rust. Frame Saver does a good enough job in coating the frame's interior that it's stood the test of time in the marketplace. There are other products that do much the same (ProGold Steel Frame Protector is one I have used). Even simple oils dripped into the frame work better then nothing.

Additional comments- The worst thing is when water gets inside without a way to get out. So tubes with single vent holes are bad. Either no vents or two per tube is the way. (This is coming from a hobby frame builder). Adding drip holes on the BB shell bottom is a common way to help this out as water will enter the frame past the seat post and through the headset. No rust preventer will last forever so periodic reapplications is a good idea, I apply Frame Saver to my bikes every few years. Andy