Dairy cow longevity affects production economy, climate footprint, and cow welfare. Based on data from the Danish Cattle Database, this research paper evaluates the relationship between early-life risk factors associated with the period before first calving and cow longevity, including data from all Danish dairy cows culled in 2019–2023. Explanatory variables for linear mixed models included calf size, twinning, and age at first calving. Information about the length of productive life (LPL) (mean: 1,074 days) and lifetime milk yield (mean: 32,088 kg energy-corrected milk) was available for 767,305 and 716,120 cows, respectively. Milk yield per day of life increased from 7 kg in cows culled during the first lactation to more than 20 kg in cows culled in their fifth or later lactations. For cows born as singletons, LPL was one month longer for cows born as large calves than for medium-sized calves, and 2 months longer than for small calves. Cows born as twins had 2 to 3 months shorter productive lives compared to cows born as singletons. For singletons, lifetime milk yield was 1,200 kg higher for large calves than for medium-sized calves, and 2,100 kg higher than for small calves. Lifetime milk yield was 1,500 to 3,500 kg lower in cows born as twins. Cows being among the third quartile of age at first calving had an estimated productive life 2.5 months longer, and a lifetime milk yield more than 2,600 kg higher than cows calving among the first quartile of age. The results from this study clearly demonstrate the importance of ‘a good start’.