Colorado is one of the top states for college graduates (38.1%), with a similar share as Connecticut, Maryland and New Jersey.
What has attracted such a highly educated population? Federal agencies? Corporate headquarters? Ski resorts?
Mountains.
If you have a boring job in a boring city, you might be willing to move to Denver and take a boring job, or even drop down below your education level if it lets you live in Denver.
Because of its growth, Colorado has a smaller 65+ population than most places (I think it is #1 in percentage of 25-65). 35 YO everywhere will have more education than 65 YO. In other places those 65 YO will still be hanging around, with very few going back to school.
Denver relatively speaking doesn't have a very large manufacturing base. It is too far for bringing in raw good, or shipping out finished products. Compare to Detroit where you could get a high-paying job at an auto plant with a HS education, or go to UM for 4-5 years, and get the same job. While there might be more life time opportunity with the degree. The immediate payback is better to start working earlier.
Denver does have a large number of regional headquarters because of its remoteness. Not all jobs require a college degree, but a college degree indicates a certain level of intelligence. You probably don't care whether your real estate agent has a college degree, but you wouldn't want one who couldn't get into college because of low SAT scores.