Studies into the effects of ageing on language in adults with Down syndrome (DS) have tended to rely on measures that lack sensitivity to change because they fail to explore across linguistic domains or rely on proxy reports. The study aim was to use measures of receptive and expressive language from studies of younger individuals with DS in exploring relationships across linguistic and associated skills, and age in young to older adults. Fifty-five adults (aged 19-58 years), 10 with a diagnosis of or signs of early stage Alzheimer's Disease (AD), provided data on measures of functioning associated with AD, non-verbal cognition, receptive language (which provided a measure of mental age), receptive and expressive language, and short term auditory and visual memory. The first order correlation between the measure of AD and CA was significant; but not when the 10 participants with AD were removed from the analysis. Significant negative correlations were obtained between CA and all other measures; small to large significant positive correlations were found amongst the other measures. Partial correlations were conducted to remove the potential effects of AD and IQ (the latter measured by a test of non-verbal cognition). Remaining significant correlations were between auditory short term memory and all other included measures, expressive language and all other included measures, and CA and auditory short term memory and expressive language. The results indicate that deterioration with age in this cross-sectional study was accounted for largely by the presence of AD. The exceptions were for auditory short term memory and expressive language. The findings may reflect an underlying deficit in auditory short term memory for adults with DS, as has been found in previous research of children and adolescents. The implications are discussed in terms of the importance of including comprehensive measures of receptive and expressive language and the need to account for the presence of AD in studies of cognitive decline associated with ageing in DS.