Not a Lego expert but from a material science perspective your worst culprit is always going to be UV, or sun exposure. It’ll change colors, make plastics britt

Not a Lego expert but from a material science perspective your worst culprit is always going to be UV, or sun exposure. It’ll change colors, make plastics brittle, and even introduce some warping. Then come heat and oxygen. Obviously some materials are more susceptible to chemical damage than others; ABS is very prone to breaking down even from exposure to weak solvents while PET is ridiculously solvent-resistant. Storing old bricks with new shouldn’t really matter. ABS off gases but mainly when heated. Most damage to ABS is irreversible. It’s not a great long-term material and it has many shortcomings.