My son is 12, learned it over the past few months, as they had little to do for school. Started simple, with basically rendering some squished spheres and cubes, then followed a few YouTube video tutorials (desk with some random stuff on it). Next, he experimented with sculpting, but still has trouble with it. I was fascinated how he took an old photo of some ship, created the outline (extruded?) then added detail. Beats computer games :)
Edit: one recommendation is to avoid books. YouTube and documentation are great.