Although the NBA doesn't have the possibility of a 19-year-old playing against a 49-year-old as we've seen the possibility for in baseball (i.e. Bryce Harper vs. Jamie Moyer), basketball isn't far behind.
Some "geezers" in their late 30s still manage to compete at the highest level with the young legs of Kevin Durant and Russell Westbrook.
The talent level from year-to-year in the NBA varies, but no matter what age you look at, there are always gifted players to be found (for the most part).
Here's a list of the greatest NBA player at each age, starting with the players who have been around the block: the veterans—or, the "old guys" who remind us of a different generation.