grand theft is a state crime, but possessing a stolen gun and being a minor in possession are federal crimes. Of course he should be in jail.
Colorado may or may not have a safe storage law.

5-8-18 Unlawful Storage of Assault Weapons.

(a) No person shall store, control, or possess any assault weapon within any premises of which that person has an ownership interest, custody, or control, in such a manner that the person knows, or has constructive knowledge, that a minor is likely to gain possession of the assault weapon and in fact does obtain possession of the assault weapon.

(b) It is a specific defense to a charge or violation of this section that:

(1) The assault weapon was located within a room or closet from which all minors were excluded by locks; or

(2) The assault weapon was stored in a locked container.

(c) It is an affirmative defense to civil negligence liability that the assault weapon was stored in a locked container.

It's quite possible the gun was secured, and the kid was simply intelligent/(un)lucky/creative enough to get through whatever security measures were in place.

You do remember the old adage about building a better mouse-trap, yes?

with tools such as paper clips, dental picks and a screwdriver. The demonstrations are freely available on the internet.

As I previously stated, there was not enough information in the article to determine how the gun was stored.

... if I steal a car and go drunk driving ... do they arrest me or the car owner?

Because if it's like you wish, I'm totally going to start stealing cars.

Where do you live?

... you were talking about the liability of law-abiding citizens when their legal property is stolen and used in illegal acts. Be it a gun, a car, a boat or a fountain pen.

Courts have established ... over, and over, and over again ... that if your property is stolen you aren't liable for what the thief does with that property.

Even in Italy.

is the ability to hold two opposing beliefs at the same time, but that's just ridiculous.

1) I own a Stack-on 14-gun cabinet. It meets the CDOJ requirements for safe firearm storage.

It is nothing more than a lockable filing cabinet. It will keep inquisitive children away from the guns (which is why I bought it), but it would not slow down anyone with a prybar (or who walks out to my garage and gets my prybar or my sledgehammer my splitting maul or...). In short, it's not going to slow down a dedicated criminal. It doesn't even have a UL15 rating which is a safe that can prevent access from an attack for 15 minutes.

2) If you are going to hold someone responsible for what others do with their guns whether they lock them up or not, why would anyone bother?

by punishing people who still followed the law?

The kids should have been taught gun safety. I was taught gun safety and nothing happened. I shot lots of game animals and saw what my gun could do and respected it.

that the thousands of kids that get shot each year or shoot others were not taught any safety rules?

...so we might reasonably assume some of those underage shooters missed valuable safety lessons due to the "abstinence-only" mindset.

no, they were not.

Defining a kid as a human up to and including 12 years old, the total of all gunshot injuries, both accidental and deliberate was 1,113.

It does appear that that particular kid didn't know what he was doing.