IIRC the thing that makes BPA plastic yellow over time is the gradual oxidation of a fire-retardant chemical added to it. You can reverse this process with bleach+UV light, reducing the chemical back to its non-oxidized form (sort of like how you can use activated charcoal to absorb moisture from the air, and then bake it to re-activate it.) If your 30yo white plastic isn't yellow, that's because the manufacturer skimped on the fire-retardant and it's likely highly flammable.

No idea about why some plastics get sticky, though. I'd imagine it's the plastic's polymer literally breaking down, but it might also be some additive "sweating" out. Maybe it's the very same fire-retardant chemical. (Old yellowing hard plastics don't get sticky, but they don't tend to have plasticizers in them, so they need far less flame-retardant.)