A number of recent studies have reported that words rated by adults as early acquired are retrieved more rapidly than words rated as later acquired. Age-of-acquisition ratings have been found to be highly reliable, but the validity of the ratings has been little investigated. This paper reports two validation studies. In the first, words were taken from a standardized vocabulary test and rated by naive adult subjects for age-of-acquisition. The adult ratings agreed closely with the rank order of the vocabulary test words, an order based on age norms. In the second study, words that had already been rated on age-of-acquisition by adult subjects were given as a vocabulary test to groups of subjects ranging in age from approximately 5–21 years. Objective estimates of age-of-acquisition from the vocabulary test data were highly correlated with the subjective age-of-acquisition ratings. In both experiments, multiple regression analysis indicated that rated age was the major independent predictor of the objective age-of-acquisition indices. It was concluded that ratings are valid measures of true age-of-acquisition.