Well, saying you can use "em" for people who are younger than you is inexact. In fact, you use "em" for people who are:
- Your younger brothers/sisters and your parents' younger brothers/sisters' children... (even if they are older than you).
- Younger than you about 10 years old or less, just younger is NOT enough, an oldman can not call a baby "em".
- Your girlfriend(s)/wife(s)/lover(s)/sexual partner(s) (even if they are older than you).
In some cases, you can use "em" to devalue a person, be careful though, also can you with closely related friends for joking only.
And of course, you call yourself "em" vice versa.