Depends on what they're made of.

Oldest boats ('70s) have PVC rudders.
Older boats ('80s) have Lexan rudders.
Some '80s boats have EPO (black epoxy fiberglass) rudders.
Newer boats could have either fiberglass (racer) or plastic rudders.

Somewhere along the line, someone could have replaced the rudders with something completely different, like wood.

Some of these rudders can be fixed, some can't. Some aren't worth fixing.

Start off with how old the boat is, what color are the rudders (have they yellowed in the sun?) and whether you can flex them with your hands (or by kneeling on them).