Some apps (like social media) have a minimum age requirement. Although it is technically possible to lie about your age to use these apps, it is not a safe thing to do.
Lying about your age means the app thinks you are older and might allow you to see/do things that aren’t suitable for your age. It also means that other people on the app think you are older and they might want to chat to you.
Don’t forget, if you can lie about your age, so can other users – they may not be who they say they are.