Look, I hear this all the time: "I want to respect my children's privacy, but I also want to keep tabs on them." OK, I get it. Kids need their privacy. But at what age does that start? In my mind, they can have their privacy at 18 years of age--because up until that time, a parent is 100 percent legally responsible for what those kids say and do. So if you condition your children early on that there is no privacy until 18, then to a degree it is at least understood that their life is an open book to Mom and Dad. Plus--let face it--as teens, we didn't always make the right choices.
http://www.huffingtonpost.com/robert-siciliano/keeping-tabs-on-your-kids_b_3963168.html