Table 1. Mean age, standard deviation and age range for all three age groups.

Age group	Mage (months)	SDage (months)	Rangeage (months)
24 months	24.31	1.16	22.05–25.96
30 months	29.81	1.49	28.16–35.22
40 months	39.69	3.52	36.01–47.97