When site factors reduce growth rates, tree lifespan tends to increase. This study investigates processes leading to such inverse relationship in Fagus sylvatica stands distributed along two elevation gradients, with an emphasis on climatic response, suppression periods, and growth trends. Dendrochronological records from old-growth beech populations sampled at different elevations within two different bioclimatic regions (Alps vs. Apennines), were used to investigate factors that control tree lifespan. Differences between old-growth (12) and nearby managed (15) stands were used to assess effects of silvicultural practices on maximum age. Logging reduced tree lifespan not only by removing older trees, but also by reducing the number of years beech individuals spent in the shaded understory. Tree lifespan and growth rates were affected by climate (spring–summer temperature) and were inversely related to one another along elevation gradients. The greatest lifespan was observed in old-growth high-mountain populations, and was related not only to slower growth due to a shorter growing season, but also to multidecadal periods of growth suppression during the initial development stages in the understory (i.e., slower growth rates at the youngest cambial ages). Past unfavorable climatic periods (in this case, the Little Ice Age) also helped increase tree lifespan. Using a linear model, we estimated a reduction in beech lifespan of 23 ± 5 years for each degree of warming. Basal area increment of trees with the maximum observed lifespan showed an increasing trend over time. Because growth of old (>300 years) trees has increased in the Alps, while it has recently declined in the Apennines, different bioclimatic regions can have opposite responses to global climatic change. In the next decades, if warming continues, beech lifespan could be reduced in the Alps by faster growth and in the Apennines by drought-induced mortality.