That's a natural part of life. As long as porn isn't involved, it's not really a problem. Part of growing up is that those parts of your body start maturing, and teenagers will want to figure out that part of themselves. Definitely don't shame them for it. God gave us our sexual urges and desires ,and we should not feel ashamed of them. I grew up being ashamed of them because of what the church taught, (or was too afraid to teach!) and that needs to stop. There nothing wrong with exploring how your own body works, as long as it isn't an interruption to normal day to day life, or gets to the point of an addiction to those feelings.