Background Few studies have examined the frailty trajectories of young–old adults using Fried frailty phenotype. Dropouts due to death were rarely taken into account. This longitudinal study aimed to identify trajectories with and without adjustment for non-random attrition and to analyse related factors.

Methods We used the first two samples of community-dwelling people in the Lausanne cohort 65+. Frailty phenotype was assessed at age 66–71 years and every third year over 10 years. A group-based trajectory modelling—first without and then with adjustment for non-random attrition—identified trajectories among all individuals with at least two observations (n=2286), excluding dropouts for reasons other than death. Multinomial logistic regressions estimated independent effects of participants’ baseline characteristics.

Results We identified three frailty trajectories (low, medium and high). Participants in the highest trajectory had a higher mortality over 10 years. (Pre)frailty at baseline was the main factor associated with adverse trajectories. Smoking, obesity, comorbidity and negative self-perceived health were associated with unfavourable trajectories independently of baseline frailty, while social engagement was related to the lowest frailty trajectory. Ignoring transitions to death attenuated the estimated effects of age on trajectories.

Conclusions Fried frailty phenotype should be assessed in individuals aged late 60s as it is strongly associated with frailty trajectories in the following decade of their life. Lifetime prevention of behavioural risk factors such as smoking and obesity is the strategy most likely to influence the development of frailty in older populations. Furthermore, our results underline social engagement as an important area of interest for future research.