Adam Harley (Kawauso)

A quick adaptation of the theme restriction code for plugins. Can only restrict on a site-wide basis, though it will only affect those sites on which it’s activated. Does not restrict super admins. Plugins must be active on the main site to be controlled by this plugin (this is an issue with WordPress’ plugin structure).
See also: <a href="https://wordpress.org/extend/plugins/restrict-multisite-widgets/" rel="ugc">Restrict Multisite Widgets</a>

Restrict Multisite Plugins

Allows network admins to restrict which plugins are available on sites, similar to themes.

Chase Wiseman

Whatever your reasons, you may need your site’s visitors to confirm their age before viewing the content of your site. Age Verify does just that. It’s a simple plugin that lets you specify a minimum age and add an age-gate between a potential visitor and your content, just to be safe. Enjoy!

Age Verify

Like my review on another similar plugin, because this plugin uses PHP to check cookies, it doesn't work with cached pages...such as with the W3 Total Cache plugin or WP Engine.

Would be better with Javascript

A simple way to ask visitors for their age before viewing your content.

Restrict Multisite Plugins vs Age Verify