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.

Anecdote about how badly ABS ages: I have a 30+ year old car and all the parts on it are fine EXCEPT everything made from ABS.

After this long inside a car (little UV exposure I'm guessing, but lots of heat exposure) the strength of 1 to 2 mm thick ABS is "empty egg shell" strength. Really really brittle.

Time to get a 3D printer!

I’ve recently unpacked my own childhoods Lego, and most pieces are like new even after 40 years. Some white pieces are a bit yellow, but it’s inconsistent, so I suspect that happened by laying in the sun before it all was packed away. The battery boxes seems to have become a bit brittle.

The lego has been stored in cardboard boxes first in the attic in my parents house, and after that in miscellaneous cellars and then an attic again.

Store tyres separately. I have tyres from the 90s that are fine. But tyres from the 2000s started melting after ~10 years. I don’t know a lot about rubber but I guess they switched from natural rubber to petroleum-based rubber. Problem with the melty tyres is that they also damage any pieces they’re touching, so it can ruin a set.