I stayed at los Alamos multiple nights and had no issues (I think it's around 6000ft as I got hit at Bryce canyon - made the mistake of staying for 2 nights at 10000ft and once I got down to 6000ft I was fine again).

DEN is 6000ft, COS is 7000 ft and Breckenridge is 9000ft. I was perfectly ok at 6000 and my altitude sickness kicked in at COS and was ok towards the end of my trip... I was at Breckenridge for 3 nights and was drinking their green oxygen water in the lobby to keep me alive.

Mr. and the kids were fine. It's just me...