It used to be for only 6months & over but when my DS was around 3months they put a new one on the shelf that could be used for younger babies.
I just went looking for it coz if DD is a bad teether I'd consider using nurofen.. but I can't find one that's for under 6 months? There's Nurofen Children, and Nurofen Infant Drops, but both are for 6+ months