Age of acquisition (AoA) is presumed to reflect the age or relative order in which words are learned, but is often measured using adult ratings or adult-reported observations and might thus reflect more about the adult language than about the acquisition process. Objective AoA estimates are often limited to words whose referents can be shown in pictures. We created a corpus-derived AoA estimate based on first word occurrences in a longitudinal corpus of child English, and evaluated its reliability and validity against other measures of AoA. Then we used these different measures as concurrent predictors of adult lexical decision times. Our results showed adequate reliability and good relations with other AoA measures, especially with parent-reported AoA (r = 0.56). Corpus AoA did not predict unique variance in lexical decision times, while adult AoA ratings and parent-reported AoA did. We argue that this pattern is due to two factors. First, the adult AoA ratings and parent-reported AoA are confounded with adult memory, lexical processing and reading difficulty variables. Second, the adult AoA ratings are related to actual age of acquisition only for words acquired during later preschool and school age. Our analyses support the utility of corpus-derived AoA estimates as an objective measure of acquisition age, especially for early-acquired words.