We can Rewrite the Stars

Phoenix D. Koa is the youngest sibling of the ASL Trio. Even though he is the same age as Luffy. it's not his fault that he was born much later in the year.

Either way, the four decided to sail away from Dawn Island after Sabo's sperm donor spots him while they are in high town. Koa explains that he would look for Sabo on the island and that their lives might be in danger. So they sail off.

Two years later, they are on the Grandline in the New World where they meet Marco. Koa decides that they should sail with The Whitebeard Pirates until they are ready to sail off. He tells Ace that he would be able to ask Roger's enemy about him and see if they would say the same thing about him as the Marines do. Ace agrees and so, they sail with The Whitebeard Pirates with Whitebeard trying to make them his sons.