Sign in to Discord on desktop and navigate to your User Settings. You can find your User Settings by selecting the cogwheel [] icon in the bottom left corner of the app.
In the User Settings menu, select Privacy & Safety.
Enable Allow access to age-restricted content on iOS by pressing the toggle. This will let you view age-restricted servers you are already a member of from your iOS device.
For Android devices:
To enable 18+ on Discord mobile, you can:
Open Discord
Click on the User Setting icon at the bottom left of your screen
Click Privacy & Safety
Scroll down, and click on the Toggle button beside Allow access to age-restricted servers