The length within 15 yards is 12.0cm, about 9-11 months

16 yards inner length 12.5cm about 12-1 3 months

The length within 17 yards is 13.0cm, about 14-1 May

18 yards inner length 13.5cm about 16-1 7 months

The length within 19 yards is 14.0cm, about 18-20 months

20 yards inner length 14.5cm about 21-2 months

21 yards inner length 15.0cm about 2 years old

22 yards inner length 15.5cm about 2 and a half years old

23 yards inner length 16.0cm about 3 years old

The inner length of 24 yards is 16.5cm, about 3 and a half years old