Please refer to below in regards to what sizing is relevant to which age groups. Please bare in mind that your childs age may not always determine their size, so please choose the most appropriate sizing relevant to which clothing size they usually wear.

Please look at each specific style listed online for sub headings / further instructions on how some pieces run in terms of comparison to average sizing. 

Size - To fit (Average)

Some pieces are of an oversized nature as is, so there will be no need to size up from their usual sizing. 

Size 0 - 6-12 Months

Size 1 - 1-2 Years 

Size 2 - 2-3 Years

Size 4 - 4-5 Years

Size 6 - 6-7 Years

Size 8 - 8-9 Years

Size 10 - 10-11 Years

Size 12 - 12+ Years

Size 14 - 14+ Years

Again, all measurements are based on "average" sizing. If you know your child doesn't generally wear size / age then please purchase accordingly to how you generally would. 
Each particular description will provide an overview of make / fit & reference the sizing on each model.