INTRODUCTION: Whether patients with early-onset dementia have poorer or improved survival compared with those with a late onset largely depends on the survival measure. Survival estimates for early-onset mild cognitive impairment (MCI) diagnosis are particularly scarce. We aimed to estimate life expectancy (LE) in patients with early-onset dementia or early MCI, and loss in expectation of life (LEL) for these groups. Comparisons were made with the general Norwegian population and a subgroup of patients with late-onset dementia. METHODS: Early onset was defined as receiving a diagnosis of MCI or dementia before age 65 years. LE and LEL were predicted using flexible parametric survival models. Our study population was comprised of newly diagnosed (incident) cases (n = 4,906), aged 50-90 years at the time of diagnosis (672 were diagnosed before age 65 years, of which 291 were diagnosed with dementia), in the Norwegian register of persons assessed for cognitive symptoms (NorCog) between 2009 and 2017, and patients were followed up for mortality or censorship until January 2018. RESULTS: Among the early-onset patients, 8 and 23% died during follow-up, in the MCI and dementia groups, respectively. Both early-onset MCI and especially early-onset dementia were associated with lower LE than in the general Norwegian population; LE for 60-year-old women in 2016 was 26 years in the general population, 20 years in MCI patients, and 7 years in dementia patients. The corresponding LE at 80 years was 10, 6, and 5 years. Thus, LEL were particularly pronounced for patients with early dementia. The diagnosis-specific LE pattern in men was similar to that in women. DISCUSSION: Early-onset MCI was associated with substantial life years lost (5-6 years), but the loss was particularly pronounced for those with early-onset dementia, reducing the expected life length by 2 decades.