Altitude!

Growing up at altitude doesn't do anything. You lose any altitude adjustment you had very quickly. People (especially children) who live at high altitude, spend a couple weeks at sea level, and then return home to high altitude, are at a high risk for severe AMS and HAPE.

I can vouch for this.  I've lived at 7200' for 15 years and do almost all my training up here.  2 weeks at sea level and I'm sucking wind on slow runs now that I'm back.  No altitude sickness tho.  I think it took just over a week to get used to it again.  We'll see at track wo tonight

OMG I get so sick from altitude! I had a race around 6000 feet and did fine. Then we went to the rockies camping at around 8500 feet. I ran a mile but felt like I had run three. It was terrible! I couldn't do much that week because I was so sick from the altitude! It takes a good couple of weeks to get used to.

As i mentioned in another thread, there are several adaptations that take place at altitude (and the reverse at sea level). Your acclimitization or deacclimatization takes place according to the rate at which each system adapts...

breathing (controlled by some chemoreceptors and your brain) adapts within minutes to hours...your breathing rate increases or decreases with a significant change of altitude in order to get more or less oxygen.

acid-base (kidneys) as your breathing speeds up or slows down, you either get rid of, or accumulate carbon dioxide, and there fore acidity/alkalinity in your bloodstream. your kidneys begin a response that takes course over days to a week or so

red blood cells - red blood cells have a life of about 90 days and their formation is stimulated by the hormone erythropoetin, which is used by dopers, as well as chronic anemia patients (frequently due to kidney disease or cancer). When going to altitude, it can take weeks to 2 months to increase the production of RBCs to a baseline steady leve. When going down from altitude, the stimulus goes away, and excess RBCs will hang out in the circulation until they are broken down by the body.

All of the above simply correlates to the DELIVERY of oxygen to your tissues (central adaptations) ...and there are other subtleties that occur as well, such has hormones that release oxygen from hemoglobin more easily at altitude (or when you are pregnant) and the total blood volume for example.

yet another facet is how the oxygen delivered is used by the body (peripheral adaptations). So if you are out of shape at sea level, and then think that you'll travel to Breckenridge to "get in shape", you'll be in less of a shape than had you done the same training at sea level due to a double whammy of not being acclimitized to the altitude, as well as your muscles not being able to use the oxygen that is delivered.

There are people known as "fast responders" and "slow responders". it's not unusual (or bad) for "slow responders" to take up to 6-8 weeks before they are feeling like themselves again after travelling to altitude. of course, that means you have to STAY at altitude that long for the adaptation to take place.