At this moment, there is no such functionality that exists on the planet to find the age & gender of a particular list of mobile numbers active on WhatsApp. We can only segment numbers based on the last seen activity, mobile operator series, or demography based numbers list. The second best option is to get TrueCaller database from us. But, that too is limited to particular set of available countries.
For best use case. Use our Last Seen Filter tool to find the most active WhatsApp numbers in your targeted list.