|Does the decreased density of the air in Denver make it easier to achieve turbulent flow at the boundary layer of a baseball and therefore make the ball fly farther?
Whew, this is a toughie. The air in Denver is less dense, so it tends to respond better to viscous forces. On that account, it would tend to be less turbulent. But it is also "thinner" and less viscous, so it would tend to be more turbulent. I think that those two effects essentially cancel, so that the ball experiences the same degree of turbulence at any altitude. However, the air in Denver has less pressure, so it exerts smaller forces on the ball than air at sea level. Thus, although the flow properties aren't affected by the increased altitude, the pressures involved are. The ball should certainly carry farther in Denver than at sea level. Imagine playing on the moon, where there's no air at all. The ball wouldn't experience any drag at all!