Increasing global policy interest in measuring and improving population wellbeing has prompted many academic investigations into the dynamics of life satisfaction across the lifespan. While numerous international projects now track adults’ life satisfaction trajectories, little research has simultaneously assessed both adults and adolescents using comparable samples and techniques. Yet adolescence harbours developmental changes that could affect wellbeing far into adulthood: adolescent life satisfaction trajectories are, therefore, critical to map and understand. Analysing data from 91,267 UK participants aged 10-80 years, sampled annually for up to 9 years, this study investigates how life satisfaction develops throughout adolescence. Using a latent growth curve approach, we find a decrease in life satisfaction during adolescence, which is steeper than at any other point across adolescence and adulthood. Further, adolescent females’ life satisfaction decreases earlier than males’; this is the only substantial gender difference in life satisfaction that emerges across the wide age range studied. The study highlights the importance of adopting a lifespan perspective with respect to subjective wellbeing in areas spanning research, policy and practice.