Users must currently be at least 13, but the firm is changing the rules ahead of the introduction of the the General Data Protection Regulation (GDPR) regulations coming into enforcement on 25th May.
The app, which is owned by Facebook, will ask users to confirm their age when prompted to agree new terms of service in the next few weeks.
It is not clear how it will verify answers from users, given the limited data requested and held by the service.
WhatsApp, which had more than 1.5 billion users in January, said in a blog post that it will not be asking for any new rights to collect personal information in the agreement.
Facebook, which has a separate data policy, is taking a different approach to users aged between 13 and 15.
It will ask the teenager to nominate a parent or guardian to give permission for them to share information on the platform.
If permission is not provided, the user will not see a fully personalised version of the social network.
More than 1bn people use Facebook Messenger each month.
Other changes announced by WhatsApp include enabling users to download a report detailing the data it holds on them, such as the make and model of the device they use, their contacts and groups, and any blocked numbers.
WhatsApp, founded in 2009, has come under pressure because of its end-to-end encrypted messaging system and its now suspended plan to increase the data shared with Facebook so adverts could be more effectively targeted.