I was born in Denver and spent my first couple years there. I remember going to back visit; Berthoud Pass took my breath away at 12,000 feet. I don't mean it was stunning, I mean there was no air, it was hard to breathe, I was getting light headed sitting in my car.
Now I've been living at sea level for years, and I feel the altitude a bit peak bagging at 8k. Nothing very serious, but exertion is harder, recovery is slower, and I start looking for the wilderness taco truck.