This study investigates the relationship between genetic predisposition to advanced biological ageing and the risk of childhood-onset recurrent major depressive disorder (MDD) in a large UK sample. Using a Mendelian randomization design, the researchers tested if shortened telomere length (TL), as indicated by single nucleotide polymorphisms (SNPs) rs10936599 and rs2736100, predicts increased risk for MDD, particularly childhood-onset MDD. The study found that the T-allele of rs10936599, which is associated with shorter TL, significantly predicted childhood-onset MDD relative to controls and adult-onset MDD cases. However, it did not predict adult-onset MDD risk. The results suggest that genetic predisposition to advanced biological ageing may increase the risk of early-onset MDD, possibly by shortening the time it takes for the disease to present itself. The study also highlights the potential of the telomerase enzyme as a drug target for preventing early-onset MDD. Future research should aim to replicate these findings in larger cohorts.This study investigates the relationship between genetic predisposition to advanced biological ageing and the risk of childhood-onset recurrent major depressive disorder (MDD) in a large UK sample. Using a Mendelian randomization design, the researchers tested if shortened telomere length (TL), as indicated by single nucleotide polymorphisms (SNPs) rs10936599 and rs2736100, predicts increased risk for MDD, particularly childhood-onset MDD. The study found that the T-allele of rs10936599, which is associated with shorter TL, significantly predicted childhood-onset MDD relative to controls and adult-onset MDD cases. However, it did not predict adult-onset MDD risk. The results suggest that genetic predisposition to advanced biological ageing may increase the risk of early-onset MDD, possibly by shortening the time it takes for the disease to present itself. The study also highlights the potential of the telomerase enzyme as a drug target for preventing early-onset MDD. Future research should aim to replicate these findings in larger cohorts.