Luffy is currently 19 years old, and under Japan's old age system, he would still be considered a "child" since adulthood began at 20. The story is currently around mid-April 1541, so we could see a timeskip to January 1542 or later in the outside world while we're still at Elbaph, a perfect way to introduce us to the current events happening around the world
In One Piece, characters age up with the new year rather than on their birthdays, following the ancient Japanese age-counting system. Luffy would be 20 by the end of the story