Steve is right about starting height and the altitude where you live in Colorado. My 4 year old daughter climbed Pikes Peak a month before turning 5. We had lived at 7,300' for a year and she had no problems with altitude or the distance climbed.
Self motivation is important and she always had no trouble getting fired up for any athletic event.
I should mention that we hiked up 6.5 miles and camped at the half way point but ascended and returned the entire distance the next day, 19.5 miles, so it was not a 1 day round trip.
