Is thumb-sucking a bad habit?

Yes: There is no benefit to thumb sucking.

Can be if it stays: Many children stop sucking their thumbs on their own sometime during the toddler years — between ages 2 and 4. For older kids who continue to suck their thumbs, peer pressure at school usually ends the habit. If lingers on , thumb sucking might begin to affect the roof of the mouth (palate) or how the teeth line up — especially if the thumb sucking is aggressive.