We use Japanese children's clothing sizes which go by the height of the child. Below we can provide an estimated age however it really depends on how tall your child is. Waist bands are elasticated.
80 - height 80cm, waist 46cm (1 yrs)
90 - height 90cm, waist 48cm (2 - 3 yrs)
100 - height 100cm, waist 50cm (3 - 4 yrs)
110 - height 110cm , waist 52cm (4 - 5 yrs)
120 - height 120cm, waist 54cm (5 - 6 yrs)
130 - height 130cm, waist 56cm (6 - 7 yrs)