A bit about a couple of my nominations.
What They Had:
When I saw this it was the most authentic portrayal of early/mid dementia that I saw. This was my mother. But that is only one of the transitions (and the least important on, in my opinion). They key is that it was also very true to what children go through in becoming parents to their parents. The two siblings were at odds. One (the one who dealt with it day by day) was ready to move to a new level. The more distant was wanting to try smaller steps. Again, this was true to my experience with siblings. There is also the husband's desire to take care of his wife as he'd promised all those years ago, without a clear understaning it was beyond his ability. 3 key steps of getting older in one movie.
Toy Story 3
As above, I don't look at the obvious getting older of Andy going off to college, but rather at the collateral issues that it raises among the toys and how that moves them to a new level of understanding/wisdom in how they will face life in a world without Andy.