We performed a statistical analysis of high-precision U–Pb zircon age distributions. This reveals that volcanic and porphyry zircon age distributions are skewed to younger ages, whereas plutonic age distributions are skewed to older ages. We show that this is caused  by truncation of zircon crystallisation by magma evacuation rather than differences in magmatic flux. Our contribution has key implications for modelling of magma dynamics and eruption ages using zircon age distributions.