Personalized I'm Nine Years Old with Dog and Mouse Youth Long Sleeve T-Shirt. Celebrate your birthday with Dog and Mouse, the unlikely best friends from the children's book "Dog and Mouse" by Michelle Nelson-Schmidt. I'm Nine Years Old with Dog and Mouse Youth Long Sleeve T-Shirt - Heather, perfect for young readers, helpers, and great friends on their birthday. youth long sleeve t-shirt. Shoulder-to-shoulder taping. Youth: S(6/8), M(10/12), L(14/16), XL(18/20)