Есть ли плагин или приложение возрастных ворот для версии 4.6?

No plugin/app required. There's an option on the AdminCP (System > Settings > Login & Registration > Registration)
On "Allow New Registrations?" option, select "Yes, with full registration form" and then below will have this option to select the age for your forums: "Minimum age required to register "

You can also activate the COPPA (Children's Online Privacy Protection Act)

 

newload I want the user to be warned about NSFW content after entering the forum, and so on.

Then the Block WYSIWYG Editor will do the trick 🤔

 

newload Yes, it can help, but it would be better if there was a plugin or application for this. Thank you.

Or creating a custom block will work too.

This video I create a Custom Sidebar Blocks (the way it was on IPB 3.x)
But the idea is the same to create ANY custom block