Personalized Superhero 1st Birthday Long Sleeve T-Shirt. Happy 1st birthday! Celebrate your baby's first b-day with this cute I'm 1 superhero design. Happy Birthday one-year-old! Pre-shrunk 100% cotton 6.1 ounce long sleeve t-shirt. Double-needle stitched neckline and sleeves. Quarter turned. Shoulder-to-shoulder taping. Adult: S, M, L, XL, 2XL, 3XL