Turbidity currents commonly transport mixtures of clay, silt, and sand, but reduced theories of depositional onset often treat the suspended load as one effective class. This obscures a central sedimentological issue: different grain-size fractions can pass from bypass to deposition at different downstream locations. Here we develop a class-dependent state-variable theory for depositional onset in polydisperse long-runout turbidity currents. Each settling class is assigned a dimensionless state variable, Bj, that compares its effective suspension capacity with its near-bed depositional tendency. The theory predicts a grain-size-ordered transition zone rather than a single depositional point. In the prescribed-background benchmark, the weak-exchange onsets remain clustered over about 13.7 km, whereas finite exchange widens the onset zone to about 174.9 km: the 125 μm boundary sand class begins net deposition near 185 km, whereas the 16 μm medium-silt class returns to the threshold near 359 km. The resulting local depositional mixture evolves from coarse rich to more mixed downstream. Sensitivity tests show that absolute onset distances depend on closure parameters, but the coarse-to-fine ordering is stable for the tested one-parameter variations and for an alternative rational suspension-capacity penalty. The framework is not a calibrated field prediction or a full hydro-morphodynamic model, but it provides a reproducible process-based bridge between suspended-load dynamics, selective bypass, downstream fining, and grain-size partitioning in turbidite records.