According to the New York Longitudinal Study, at what age can infants' temperaments first be classified?

According to the New York Longitudinal Study, at 3 months of age can infants' temperaments first be classified?.

Explanation:

New York Longitudnal Study includes the investigations of the direct observation and interviews with the parents about their children.

It was started in 1960s.

By 3 months they found four categories of temperament:

1) Easy

2) Difficult

3) Slow-to-warm-up.

4) Hard to classify.

Hence According to the New York Longitudinal Study, at 3 months of age can infants' temperaments first be classified