Prince Louis is celebrating his 8th birthday on the water. "Happy birthday, Louis!" the official Prince and Princess of Wales Instagram account captioned a photo of the young royal on Thursday, April 23. "8 today!" Prince William and Princess Kate Middleton's youngest son was all smiles as he stood with his arms crossed with a body of water surging behind him. Louis wore a dark blue knit half-zip in the image. Prince Louis Matt Porteous Royal watchers couldn't help but comment on how much Louis looked like his older brother, Prince George.