Personalized 4th Birthday Mermaid Tail Girls Youth Long Sleeve T-Shirt. 4th birthday Youth Long Sleeve T-Shirt - White party idea has starfish and mermaid tail with this mermaid is 4 message youth long sleeve t-shirt. Shoulder-to-shoulder taping. Youth: S(6/8), M(10/12), L(14/16), XL(18/20)