Former commander of the Whitebeard Pirates, Marco, debuted in Chapter 234 of the One Piece manga and Episode 151 of the anime. He started out as an apprentice and climbed the ranks to become Whitebeard’s right-hand man. After Whitebeard’s death during the Summit War of Marineford, Marco assumed leadership of the Whitebeard Pirates. He led the crew until they disbanded following the Payback War.
After retiring from his years of piracy, Marco became a doctor in Whitebeard’s native village of Sphinx. Marco has been a major ally to Nekomamushi, Jinbe, and the Straw Hats during the Marineford and Wano Country Arcs.
When we first meet Marco, we can conclude that he’s much older than Luffy, but his appearance doesn’t indicate any particular age. Upon first glance, Marco could be considered any age between 25 and 50, depending on one’s own perception of him.
Nicknamed “Marco the Phoenix,” the former captain of the Whitebeards debuted as a 43 year old. After the timeskip, which occured between Chapters 597–598 and Episodes 516–517 and turned Luffy from 17 to 19, Marco himself had two years added to his age, which would make him 45 currently.
Similar to how Luffy gained rubber-like properties from the Hito Hito no Mi: Nika (Human-Human Fruit in English), Marco ate the Tori Tori no Mi (Bird-Bird or Tweet-Tweet Fruit) that allows him to transform into a phoenix and a human-phoenix hybrid at will.
He has assisted the Straw Hats on numerous occasions, namely in the battle against the Big Mom Pirates, including Big Mom herself. This occurs in Chapter 995 of the manga and Episode 988 of the anime.
