We have always charged our children rent as soon as they were earning a wage. We charged a 1/3rd of their wages as this is roughly what they would pay out for rent. At the time they complained and felt it was harsh but always paid (standing order set up) but now they have left home they all say that this helped them as they didn't get used to having all their money for themselves and moving out wasn't too much of a shock financially.

If they didn't like it they could move out at any time!