Lying about your age when dating – whether on or offline – is not unusual, and sites may or may not wish to prevent this. Indeed, dating sites could still allow it, even after applying age verification for harm prevention purposes. That would be a matter of policy, and marketing considerations.
But a further benefit of applying AV to all ages may be that fraudulent users would think twice if they knew they had to, at some stage in the profile creation process, verify their age by disclosing their identity – albeit to an independent third party who would not share personal details except an age confirmation with the dating sites. This could therefore help tackle dating fraud, and indeed, be a further deterrent to those who seek victims for non-financial crimes.