The internal heat flows of both Uranus and Neptune remain major outstanding problems in planetary science. Uranus’ surprisingly cold effective temperature is inconsistent with adiabatic thermal evolution models, while Neptune’s substantial internal heat flow is twice its received insolation. In this work we demonstrate that convective inhibition by methane condensation efficiently decouples these planets’ photospheres from their interiors. In the case of methane, this decoupling allows the atmosphere to rapidly cool while the interior remains hot. This effect can reduce the thermal age of Uranus or Neptune inferred by their luminosities by nearly 50%. If water condensation inhibits convection by a similar mechanism deeper in the atmosphere, the effect is in the opposite direction. In the case of water, convective decoupling causes the interior to lose heat less efficiently, slowing the thermal evolution process and increasing the thermal age of Uranus or Neptune by more than 50%. If convective inhibition by condensation indeed occurs in Uranus and Neptune’s envelopes, then it is an essential consideration to understand their thermal histories and contemporary heat flows.