Snapchat asks for your birthday when you sign up as part of there efforts to keep children safe.
The official terms of use state that you have to be over 13 years old to use Snapchat, and requesting your birthday when you sign up is one way the Snapchat team can work out who is too young to be using the service.
What Snapchat does with details about your birthday
There are two main reasons Snapchat asks for your birthday:
1. To make sure all Snapchat users are over 13. This is a legal obligation, so they have to do this.
2. It provides an opportunity for the brand to communicate with users and boost engagement through birthday messages etc...
Some social networks display your birthday to encourage people to wish you happy birthday through the app. This can increase engagement, so creates an incentive for apps like Snapchat to display your birthday.
Snapchat launched an initiative called Snapchat Kidz, which is tailored for under-13s.
You (or your parents) can read more about it in the Snapchat parental guide: http://www.snapchat.com/static_files/parents.pdf
