BACKGROUND: An important 'window of opportunity' for early life exposures has been proposed for the development of atopic eczema and asthma. OBJECTIVE: However it is, unknown whether hay fever with a peak incidence around late school age to adolescence is similarly determined very early in life. METHODS: In the PASTURE birth cohort potentially relevant exposures such as farm milk consumption and exposure to animal sheds were assessed at multiple time points from infancy to age 10.5 years and classified by repeated measure latent class analyses (N=769). Fecal samples at age 2 and 12 months were sequenced by 16S rRNA. Hay fever was defined by parental reported symptoms and/or physician's diagnosis of hay fever in the last 12 months using questionnaires at age 10.5 years. RESULTS: Farm children had half the risk of hay fever at age 10.5 years (adjusted odds-ratio (aOR) [95% CI]=0.50 [0.31; 0.79]) compared to non-farm children. While early life events such as gut microbiome richness at age 12 months (aOR=0.66 [0.46; 0.96]) and exposure to animal sheds in the first three years of life (aOR=0.26 [0.06; 1.15]) were determinants of hay fever, the continuous consumption of farm milk from infancy up-to school age was necessary to exert the protective effect (aOR=0.35 [0.17; 0.72]). CONCLUSION: While early life events determine the risk of subsequent hay fever, continuous exposure is necessary to achieve protection. These findings argue against the notion that only early life exposures set long-lasting trajectories.