  How does a steady state with strong intermittency develop in time from an
initial state which is statistically random? For passive sliders driven by
various fluctuating surfaces, we show that the approach involves an
indefinitely growing length scale which governs scaling properties. A simple
model of sticky sliders suggests scaling forms for the time-dependent flatness
and hyperflatness, both measures of intermittency and these are confirmed
numerically for passive sliders driven by a Kardar-Parisi-Zhang surface. Aging
properties are studied via a two-time flatness. We predict and verify
numerically that the time-dependent flatness is, remarkably, a non-monotonic
function of time, with different scaling forms at short and long times. The
scaling description remains valid when clustering is more diffuse as for
passive sliders evolving through Edwards-Wilkinson driving or under
antiadvection, although exponents and scaling functions differ substantially.
