I was wondering if anyone has opened a sealed set like Taj Mahal or Star Wars UCS Imperial Shuttle after it had been sealed for years, and if so, were the white LEGO elements still perfectly white, or did they fade or yellow even while sealed? I'm debating about buying an expensive UCS Imperial Shuttle that's still sealed, instead of Bricklinking, just so I can have one looking brand new, but wanted to know if it might already have faded or yellowed even while sealed.
Lego bricks typically turn yellow due to a chemical change that occurs when UV light interacts with flame-retardants in the plastic. So bricks stored in darkness shouldn't turn yellow at all.
Should be Fine
The sealing process, at least in the new bags without perforation, keeps the bricks safe from aging. The Taj Mahal should be just fine, and even opened, my Imperial Shuttle bricks are great!
In fact, all the newer bricks, 2003-2007, seem to be pretty yellow-proof. You'll typically only find those in much older sets.
Yellowing occurs because of UV interaction with the bricks. Box, indoors, away from windows, any of the above should keep them plenty safe.