The last one is only good if you actually read it. I might end up seeing the first clause, assuming the entire line is irrelevant, and moving on without seeing the "Otherwise".
Prefer declarative statements:
"If you are 60 or older, you must fill form 445.
If you are 59 or younger, you must fill form 331."
Prefer declarative statements:
"If you are 60 or older, you must fill form 445.
If you are 59 or younger, you must fill form 331."