We would recommend that you pick up an airsoft gun for someone who is in their teenage years, as they will have already developed the complex motor skills and hand-eye coordination necessary to make the more complicated shots.
You can give a smaller airsoft rifle or BB gun to a child, especially if you plan on developing these skills a lot quicker or harbor secret desires for them to accompany you on that all-important hunting trip. However, you’ll need to make sure that any child with a BB gun is under strict supervision, as they could fire it at exposed skin or eyes, which could be very dangerous.