For example by photo analysis or maybe you partnered with some third-party provider for this purpose? If I’m going to make some social network based on your services, I would need to verify users age somehow.
You can look at the policies and technologies being used by other companies as a starting point.
For example, Epic Games is one company that offers various age verification methods.
For general adult verification without compromising user privacy, Yoti seems pretty good.
But in the case of Yoti I think this works well because it can determine with pretty high confidence “is this a parent or a kid?”
When you need to know “is this person 18 years old or 17 years and 364 days old?” - for example - something more precise involving IDs may be needed.
What are your legal obligations as the developer? What approach offers a good balance to deter fraudulent (underage) registrations without inconveniencing your legitimate customers too much?
These are probably questions to ask yourself first before asking the technical ones.
So once you look at the various age verification services out there, you can ask a more specific question to the Backendless Team, like: “how to I implement x service in Backendless?”