Watch: How the Royal Family Honored Queen Elizabeth II on 100th BirthdayThis photo is royally sweet. When Prince Louis-the youngest of Prince William and Kate Middleton's three kids, in addition to Prince George, 12, and Princess Charlotte, 10-celebrated his eighth birthday on April 23, his parents marked the occasion by unveiling his annual birthday portrait on social media. In the image, a smiling Louis stands in front of an ocean backdrop, his arms crossed over a blue knit quarter zip.