Warning to view photos? Please elaborate what you're trying to achieve, on what page this warning should be? A screenshot would be great. 

It should appear on the bulletin board and browse the gallery of photos.

To add a module age restriction for the photo uploading by users.

This is a custom code  modification. 

how to proceed?

You need to hire a programmer to do that for you or do that on your own. 

You can also make a post in the custom code modification section of the forum, probably some community member will assist you to do that. 

I put topics on the forum to help to make this change