Telomere length is a reliable biological marker that estimates ageing in people. They are the protective ends of chromosomes that are composed of DNA and protein. When the telomeres get too short with age, it signals the cell to stop dividing causing cellular senescence and hence death. The aim of this study is to establish if there is any solid association between cognitive health and telomere length. This will be further beneficial for early detection and monitoring of cognitive death. Previous studies done in this area found positive associations, while others showed no clear link. One limitation found in previous studies were less number of participants involved hence this study took almost 8000 participants from European background and tried to explore whether measured telomere length (RTL) and genetically predicted telomere length (PGS-TL) are associated with baseline cognitive performance and cognitive changes over time. Methodology Initial data was drawn from 7,877 participants in the PROTECT study, a UK-based longitudinal research initiative focused on aging and cognition. Inclusion criteria was participants should be dementia-free, aged ≥50 years and had access to computers and internet. Data collection was further done in two forms- Relative Telomere Length (RTL): Measured from DNA extracted from saliva samples of 846 participants and Polygenic Scores for Telomere Length (PGS-TL): Computed from genome-wide data for all participants. Cognitive performance was assessed using tests-Paired Associate Learning, Digit Span Test, Self-Ordered Search, Verbal Reasoning. Age, genetics, education level, sex, employment status, taken as covariates. Latent growth models were used to assess cognitive performance from the data of participants. Results The sample was further divided into two groups based on the median age (~62 years) to compare results for middle-aged vs. older adults, No strong or consistent relationships were found between telomere length (RTL or PGS-TL) and baseline cognitive performance or its changes over time. In case of more than 62 years old adults, Longer RTL was weakly associated with worse verbal reasoning performance at baseline, though this finding did not survive statistical correction. Higher PGS-TL predicted slower cognitive improvement in verbal reasoning initially, followed by more rapid improvement after two years. Significant associations were nil between telomere length and cognitive performance when observed in middle aged adults that are 50-62 years old. It was established by the researchers that cognitive scores tend to get improved over time, likely due to the repeated exposure of the patients to cognitive tests rather than genuine cognitive changes. Conclusion It is the first study in this area to examine the relationship between telomere length and longitudinal cognitive performance throughout life, using both measured (RTL) and genetically predicted (PGS-TL) telomere length using a large amount of data set. It is concluded that both telomere lengths do not appear to be a strong predictor of cognitive performance or age-related cognitive changes in older adults. The study emphasizes the need for alternative biomarkers, such as epigenetic clocks, for better prediction of cognitive decline. Overall, the results drew into question the utility of telomere length as an ageing biomarker that is capable of predicting cognitive performance and age-related change, in typically ageing individuals but other alternatives can be used as well. Impact of the research The study challenges previous assumptions that telomere length is a strong predictor of cognitive performance and decline. It suggests that the link between telomere biology and cognitive aging may be weaker or more complex than expected. It emphasizes the need for longer-term studies and more diverse populations to capture the complex dynamics of aging and cognition and promotes the integration of other variables like environmental, genetic, and lifestyle factors in future research on cognitive health.