I would suggest getting your teenage daughter to read up about fasting. It can help with mental illness there's no doubt about that. There's also little doubt that a bad diet can make mental illness worse even if it isn't a contributing factor in it's cause from the start. She is too old to 'put on a fast' though, she would have to make the choice for herself.
There is a Depression Forum on Curezone that gives advice on natural ways to deal with it. Chris is active there and has probably given you some good advice about it already.
As for how young we can fast. I've read about an 11 month old baby that was successfully fasted for about 8 days after being overfed on formulas and excess food. Fasting is natural and if the body needs to fast it will remove any appetite, the only reason we don't feel the 'need' to fast usually is that we are fed with inferior foods which make us want more all the time, because they are not nourishing our body properly. I brought all my children up on fruit only from 6-12 months. They never spat their food across the room as so many do with 'baby foods'. They ate their fill and left it when they had had enough, not under or overeating. It is only upon introducing artificially constructed foods that dietary problems occur.