Age the stock, not the print
Colour film carries three sensitive layers, and they do not decay at the same rate. As a roll passes its date the blue-sensitive layer usually fades first, then green, then red. The result is not a uniform tint: shadows drift one way and highlights the other, a split that photographers call crossover.
Degradation is the master control. Everything below scales with it, and at 0% the image passes through untouched.
Colour crossover
Shadows and highlights receive opposite tints, interpolated across the tonal range by luminance. Positive values give magenta shadows against cyan-green highlights, the standard signature of expired colour negative. Negative values invert that.
Crossover is the reason expired film cannot be corrected with a single white balance move. Fixing the shadows breaks the highlights, which is exactly what the split here reproduces.
Base fog
Silver halide develops slowly even with no light reaching it, and that accumulates over years. The result is density across the entire frame, including the parts that should be clear.
Base fog raises the black point up to about 52 levels and rescales the tones above it. That flattening is separate from the contrast loss that Degradation applies, so both together produce the washed, low-density look of a roll stored in a warm cupboard.
Emulsion mottling and grain
Grain works pixel by pixel. Mottling works in patches roughly 14 pixels across before smoothing, modulating both exposure and cast so some regions look further gone than others.
Real damage from heat and humidity is uneven, and a frame degraded perfectly evenly reads as a filter rather than as film. Mottling is what breaks that regularity.
Example: portrait on a 10-year-old roll
Start at 62% degradation, 38% crossover, 40% fog, 34% mottling, 36% grain. Skin should hold a magenta cast in the shadows while highlights cool off. If the face looks sunburnt, lower crossover to around 20%. If the frame reads dirty rather than aged, cut mottling first, then grain.
Backlit scenes exaggerate fog because there is little dense shadow to anchor the range. Reduce it to about 20% there and let crossover carry the effect.