I think it is a type of elm. Chinese and siberian elm are two kinds around, not sure how much american elm is around. Or could be a hybrid?

Here is a link that might be useful: {{gwi:323710}}

Considering that the only Elm that is naturalized(there are no native Elms in Colorado) in your area is Siberian Elm (Ulmus pumila), I bet it's most likely that species. Although American Elm (U. americana) has been commonly cultivated in Colorado and that's possible. But your seedling looks more like Siberian Elm to me.

Also, Siberian Elm has widely erroneously been called Chinese Elm, the true Chinese Elm(U. parvifolia) is mostly sold as Lacebark Elm and usually has very ornamental multi-colored bark which is completely unlike any native Elm and the vast majority of cultivated Elms. It's also only been in wide spread cultivation for about the past 20 years or so.

Looks like a Zelkova to me, a fine bonsai subject and specimen tree in its iown right!

It is some kind of elm. The veins on an elm leaf are always perfectly parallel, just like those. No other tree leaf has perfectly parallel leaves.