1st Position Asymmetric Sequin Lyrical Unitard with Mesh Panels - All Questions

On size guide it only gives you age of child at the top, but not sure which size dress to go for. Our daughter is 7yrs and says she would be medium but not say medium on options , just x child, xx child xxx. Fluid you tell ne pls. Also, what’s earliest I could get this costume by as need ASSP. Many thanks