This is a delicate question. It is not a subject that I can ask my children about.
At what age can a parent expect this activity to start?
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It depends on the kid actually. Many kids will "play" even as toddlers and in elementary school and just experiment with the different sensations that happen in that area of the body. However, most kids hit puberty and start masturbating with an actual result sometime during the Middle-School years or between 11 and 15. Of course there are exceptions that start a little sooner, or who might hold off and not masturbate at all (for a variety of social/religous reasons, not neccessarily because they don't get the urge).
I taught Middle School Health (which includes sex-ed) and actually would suggest that if there is a concern you have or something going on you think needs some work (for example, maybe you are finding stains on sheets and would like to talk about how your son can wash up after himself or are thinking it is time he start washing his own bedding) I'd actually suggest finding a way to brooch the subject (as awkward as it may be). If there is some other reason for not talking about it besides the usual awkwardness, of course the following paragraph need not apply.
I know you have both a daughter and a son that fall approximately around this age category so I'll use he, but she can fit too. If you can just be matter of fact about it, state that you don't want to impede on his privacy or make any one feel ashamed but, then state what you noticed and follow it with what you would like to see happen to remedy the problem. If you keep it short and simple and in a non-emotional tone. As embarassed as your kid will be, he is likely to respect you for your frankness later on and your openess will make it more likely he will feel he can talk to you about sexual matters when he has questions, concerns or experiences he needs to discuss with you later on.
Teenagers? I've masturbated for as long as I can remember, probably since age 3 even though I didn't know what it was - it just felt good. As the years passed I learned what it was and just kept on going. Always in private, but at some point my awkward mom asked me to not stain the bedding. I complied and there hasn't been a word since.
This varies dramatically from kid to kid. Some discover it as toddlers and never forget, others not until well into their teens. Because the libido spikes around puberty, this is the most common time to start regular masturbation (usually as a tween), especially for boys. A lot of young women don't start masturbating regularly until a bit later due to a variety of social and physical reasons, but many/most are around this same time.
Again, varies dramatically.
Fortunately, as parents, our childrens' masturbation should affect our lives about the same amount our masturbation affects theirs.