We studied the age-related differences in inspection time and multiple cognitive domains in a group of monozygotic (MZ) and dizygotic (DZ) twins aged 7 to 17 years. Data from 111 twin pairs and 19 singleton siblings were included. We found clear age-related trends towards more efficient visual information processing in older participants. There were substantial correlations between inspection time and cognitive abilities. The heritability of inspection time was 45%, and ranged from 73% to 85% for cognitive abilities. There were significant non-shared environmental effects on inspection time and Wechsler IQ scores, but no shared environmental effects. The genetic correlation between inspection time and Performance IQ was 0.55 and with Verbal IQ it was 0.28. There was a significant non-shared environmental correlation of 0.24 between inspection time and Verbal IQ.