I was first given one of these at the age of 11, I've been doing them ever since (I'm now 26). I find they're a great way to destress and calm my mind after a long and stressful day at work. They take an immense amount of concentration and patience to do well but the result is well worth the effort. The only thing I think I could say against them is that once completed they will soon become tatty if not stored correctly.
I think these are great, anything that encourages concentration and patience is always good. An enjoyment of art in general is a good starting point before getting one of these but they're a fairly cheap and enjoyable hobby. They kept me quiet for hours as a child / teenager and are still something I find enjoyable in any quiet time I get. I will be introducing my daughter to them at some point.