Personalized 4th Birthday Mermaid Party Youth Long Sleeve T-Shirt. Cute 4th birthday mermaid Youth Long Sleeve T-Shirt - Royal Blue with a blue mermaid tail for a 4 year old party photo. youth long sleeve t-shirt. Shoulder-to-shoulder taping. Youth: S(6/8), M(10/12), L(14/16), XL(18/20)