Personalized Birthday Stamp 21 T-Shirt. 21st birthday T-Shirt - White with distressed canceled postage stamp with fun birthday cake and number 21. Everyone loves our Custom T-Shirts! Wardrobe essential for looking good, 24/7. 5.0 oz., pre-shrunk 100% cotton. Seamless 1x1 rib collar. Shoulder-to-shoulder taping. Double-needle stitched sleeves, bottom hem and front neck. Note: greys are a cotton/polyester blend.