As you probably realize, putting an age on an activity is not an exact science. Under 3 years old, however, is a safety issue since humans that age have a tendency to put things in their mouths that ought not be there, so please be aware of this.
Other age ranges generally get into how much a child will be willing and able to do with whatever project is before them. This varies greatly between kids, so make a judgement call and go for it. If it's too hard, maybe they'll like it later. If it's too "young" for them, they may get bored easily. It's hard to say.