I learned how to paddle when I lived in Boulder. I learned on Boulder Creek in town. I paddled from the whitewater park down to the park at 30th street several dozen times that summer.
It is mostly non-threatening class III with enough rocks, trees, and man-made stuff like the diversion at Broadway to make it very exciting for a beginner. Be careful at high water (water up to the sidewalks) because the water is in the trees at that level and shouldn't be attempted by a solo beginner.
As soon as it looks like it will float a boat you should start because you will want to get some practice before it jacks up to high levels. When you are a beginner in a decent snow year it seems like all of a sudden everything got huge and there is nowhere close by to paddle that isn't scary....so get it early.
If you want someone to take you down for your first few times send me an email: firstname.lastname@example.org