How can you tell if someone is as mature as their age assumes? Well, there is only one way I know how. And that is through conversation. You can tell a lot about someone through conversation.
When you speak to a person and listen closely to what they have to say, you can learn a lot about them. You just have to listen to what they say and study their thought patterns and you will be surprised at how well you can assess their maturity.
It is something I do on a regular basis because I sometimes encounter situations that need me to be sure of someone's maturity before committing myself to an end. It comes with time and practice, not to mention patience.