Well, when you say "legal" that would seem to include both criminal and civil consequences.

From the criminal perspective, it depends whether it's a handgun or a long gun.

CRS 18-12-108.5 makes it illegal for a minor to be in possession of a handgun unless " on real property under the control of such person's parent, legal guardian, or grandparent and who has the permission of such person's parent or legal guardian to possess a handgun."

I underlined "under the control of" because there could be a question of whether your property is "under your control" when you aren't physically there - some might say that if you aren't on the property, that property is not "under your control" and therefore you cannot permit a minor to possess a handgun there.

There is also a narrow exception carved out for a minor who keeps a handgun with the permission of the parent or guardian solely for the purpose of self defense.

As long as you are physically present on the property at the same time the minor is in possession of the handgun, I see no criminal issue.

There is no Colorado law that prohibits a minor from being in possession of a long gun.

Now, if you're talking about civil law, that's where things get murky. As has already been said, you may trust your kid, but what about his/her friends? Do you know them? Do they know you have firearms? Kids do stupid things, even smart kids. If some other kid finds out your kid has a firearm and then when you're not home, comes over to the house and engages in some kind of horseplay and gets injured or killed (or injures or kills someone else) you could absolutely be liable for negligence.

Remember also that unlike criminal court, where the standard or proof is "beyond reasonable doubt", in a civil suit, the standard of proof is "a preponderance of the evidence" or 50% +1. That's a low standard of proof and if you are being sued by the family of a dead or maimed kid, your chance of avoiding liability is going to be slim if you knowingly allowed your minor child to access your firearms without you physically being present.

Standard disclaimer: This does not constitute legal advice, if you want real legal advice you should consult a lawyer on your own.