A compound sentence needs a comma before the conjunction.
I’ve been to nine planets in twelve years, and it’s starting to show.
You can tell this is a compound sentence because you could delete the conjunction and make two smaller sentences: I’ve been to nine planets in twelve years. It’s starting to show. The flow changes — becomes slightly choppy –when you do that, but that may be exactly what you want.
Kudos to the person who created this example for spelling out the numbers and for using the correct form of it’s.