On a computer:

1. Open YouTube.

2. Click on your profile picture in the top-right corner of the screen.

3. Select "Settings" from the drop-down menu.

4. Click on "Restricted Mode" in the left-hand menu.

5. Toggle the switch to "On".

On a mobile device:

1. Open the YouTube app.

2. Tap on your profile picture in the top-right corner of the screen.

3. Select "Settings" from the drop-down menu.

4. Tap on "General" in the left-hand menu.

5. Scroll down and tap on "Restricted Mode".

6. Toggle the switch to "On".

Please note: Locking YouTube will not prevent users from accessing other websites or apps. If you want to restrict access to other content, you may need to use a parental control app or service.