Very much dependent on age, rest and general conditioning. I went from sea level to 14K at Pikes peak in 1 day and it was quite uncomfortable. I managed, but folks who lived in Denver with lower physical fitness levels than me, did better.

Agreed, we live at ~5K and went up to Pikes Peak; my wife and I had no problems (beyond minor headache), but my son's lips were turning blue and he was feeling pretty bad.

Other amusing things from that trip: we went up there the 3rd of July, and it snowed. We charged the car in Colorado Springs before we left, got up to the peak with 36% battery remaining. My wife worried we wouldn't be able to make it back. Got back to CS with ~70% battery left.