Depreciation is for 25 years, not 35. They hold the same book value after that point and capital expenses like D checks are immediately expenses instead of depreciated. Game doesn't let them fly past 35 years. There's no good reason for the limit to exist other than it being a game play decision by the powers that be.
However, by the mid to late 20s in age, maintenance costs go parabolic, so by the time you get to the mid 30s in age, maintenance costs are untenable - you can often buy a 20 year old example of the type for less than the cost of heavy maintenance on a 34 year old plane. I personally disagree with the maintenance curve going like that, but that's how the game works.
Cliff notes: limit is 35 years for flying but it becomes uneconomical before that due to maintenance costs.