@bythewell - Well, to be fair, those are only estimates and they are probably based more on how much flight time a plane gets than simply how old it is. Landing gear that's used twice a day is going to depreciate much more quickly than gear that's used twice a year.
And everything is worth only what you can get for it, after all. If you have a perfectly good set of landing gear that doesn't show any real wear and tear, but is older than the estimated age of use, well, you might not be able to sell it for much, but you'll still be able to use it. It might not count as a financial asset, but it's not a loss either.
It's a good idea to have a depreciation schedule, because that means if you need to replace it, you'll have the funds ready, but I don't think they get replace regardless of if they need to be.