I think none of those is the best way to write this. Start with the conditional first, so the reader knows a branch is coming. Then have the most common case immediately following. Invert the conditional if that's necessary to put the common case first. Restating the opposite of the conditional can make it even clearer.

"If you are age 59 or under, fill form 331. If you are 60 or over, fill form 445."