The growth of the spin-glass correlation length has been measured as a function of the waiting time tw on a single crystal of CuMn (6 at.%), reaching values ξ ∼ 150 nm, larger than any other glassy correlation-length measured to date. We find an aging rate d ln tw/d ln ξ larger than found in previous measurements, which evinces a dynamic slowing-down as ξ grows. Our measured aging rate is compared with simulation results by the Janus collaboration. After critical effects are taken into account, we find excellent agreement with the Janus data.