I am not a lawyer, and do not play one on TV.
So the following is worth exactly what you paid for it.

From the BATF FAQ

(F6) Does a customer have to be a certain age to buy firearms or ammunition from a licensee?

Yes. Sales of handguns and ammunition for handguns are limited to persons 21 years of age and older. Although some State and local ordinances have lower age requirements, dealers are bound by the minimum age requirements established by the GCA.
As you know, you must be 21 to buy from a dealer. What this quote references is that the BATF recognizes that some states have lower age requirements, than imposed by the guideline for dealers.

Colorado revised Statutes does specify an age limit.
If you HAVE attained the age of eighteen years.......well....

18-12-108.5. Possession of handguns by juveniles - prohibited - exceptions - penalty.

(1) (a) Except as provided in this section, it is unlawful for any person who has not attained the age of eighteen years knowingly to have any handgun in such person's possession.
Now.......will a LEO take your handgun, impound your car, and throw you in jail, if you are found to be under 21, and in possession of a handgun.........?

LEO's do NOT always know the law. They act. They write tickets. They let the courts decide. Will you be out hundreds, possibly thousands of dollars, only to find you were RIGHT to begin with...........maybe, maybe not.

I would print out the C.R.S. and carry them with you, any time you have firearms in your possession.