[QUOTE=ForresterModern;769592]
Any 14er is quite an accomplishement at any time of year as they generally remain snow covered the year round, and the air becomes dangerously thin of O2 after about 11000 feet, making every foot step upward exponentially harder, without supplemental O2.
QUOTE]

I am afraid I will have to disagree with these two points. Most if not all of the snow is gone from all 54 14ers by late June. The only glacier [i.e. permanent ice field] in Colorado is acutally quite low in elevation. The climbing season for most people is June through the end of September.

There is a point where the O2 level gets too low but it is closer to 18 or 19K not 11. If it were only 11 K the entire nations of Bolivia, Nepal and large chunks of Peru and Mongolia would be in trouble.
As happened to Mrs. IrishColoradan there is always a risk of altitude sickness no matter how long you have been at relative altitude.

I climbed all of the 14ers before I was 16 and had about 80 of the 100 highest in the state done before I left for college, have not climbed any since . My father was well on his way to the top 300 when he died and I ahve a friend who is one of ~10 people who have climbed all of the 13ers in Colorado [~380 peaks]