Can your twelve year old son watch his four year old sister?

It depends on the kids ... mostly. I did - but that was 60 years ago. (yes, we're still on speaking terms with each other).. If you mean for long periods of time, then no. (MORE)
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Can twelve year old watch m movies?

The rating M stands for mature 18 and older. So you can only watch it if your parents think it is appropriate for you and it is not in a public theater.