"I received a photo of a stranger in the mail" or "I'm being chased by colleagues in a social networks" can be warning signs of security problems. Do you know how to proceed?
The use of strong keywords, combinations of letters and numbers without recourse to the more common "password" and "123456789" is one of the first recommendations for online security at all ages. It may be difficult to realize to the younger that you must have a password that is difficult to crack, and that this should never be shared with friends, even the close ones, but this is a first step in protecting yourself.
Probably most people explain to their children that they should not talk to strangers on the street. The same can apply to the on-line world, even if the "character" on the other side of the screen appears to have the same age and preferences. Unfortunately it is usual that people with less clear intentions hide behind friendly "appearances". However try to not be so strict, on-line friendships can occur - I do have a couple of friends that I first met on-line.
One of the most frequent problems of identity theft happens using public computers, where the sessions in social networks and email accounts are active after the user finish his/her task. Even with time limits imposed on some platforms for automatic closure of the sessions, the most appropriate procedure is to always manually "log-off".
Yes... it seems too much work having to enter a password - or a code of gestures - every time you use your phone, especially if you use it every 2 minutes, but this is one of the safest ways to ensure that your information does not fall into unwarranted hands. And by the way, you should also add password lock on tablets and to the laptop screens. Here apply the same rules.
Have you explored the security settings of the browser, search engines and social networks? There are some simple steps you can do to prevent your data and images to be shared publicly, normally this is done in the definitions of groups of friends with whom you can extend ties of greater confidence.