At any age it is affected by this concern: because it was the habit for a long time (like the cigarette), or – and this remains the most widespread – when we are in a period of stress: frustration, fear, fatigue, etc.
Sometimes this habit affects people who hyperactivity. Those who need to stir constantly or do not stop bite their nails when they bother: still in the classroom or in front of a computer for hours for example.