My almost six-year-old daughter has been chewing almost anything since infancy (we adopted her when she was 7 months). She has always bitten her finger and toe nails (we've never clipped them), and she also chews her hair, clothes, jewelry, toys, stuffed animals, found objects, broken glass, etc. She does not tend to swallow what she chews.
We do not scold her when she chews, but we do ask her to chew something safe if she picks up something unsafe (e.g. chew a toothbrush instead of jewelry). I cannot correlate her chewing with stress; she chews at bedtime, while reading, in front of the TV, in the car, at kindergarten, etc.
Her speech was delayed and her pronunciation is still behind her peers (her language and vocabulary are at or ahead of age level). Our speech therapist told us that chewing is common amongst kids with delayed speech - they seem to be seeking oral stimulation, perhaps to better develop their use of their mouth and tongue.
That said, I'm still concerned. Should we be looking for a potential vitamin or mineral deficiency, or is it more likely this is just a confirmed habit. We have read that we should stop her nail biting for health reasons, so we are trying to figure out what steps to take next. Should we address the nail biting only, encouraging her to chew other things instead, or perhaps offer chewing gum on an ongoing basis?