| AGE IN MONTHS | SOURCE |
|---|---|
| 11.2 months | Sutherland, Olshen, Biden, & Wyatt, 1988 |
| 11.3 months | Denver Developmental Screening Test (1967) |
| 11.7 months | Bayley Scales of Infant Development (1969) |
| 12 months | Revised Denver (1981) |
| 15 months | Nelson Textbook of Pediatrics (1987) |