For security and functionality, consider upgrading to a device running at least iOS 12 or newer. If you must keep iOS 5.1.1 for nostalgia or specific legacy apps, use the web browser for YouTube. Sideloading untested IPAs from the internet is generally discouraged due to lack of updates and potential malware risks.
To get the legacy YouTube app working again, the device must be modified to bypass modern security and API hurdles. youtube ipa for ios 511
Find and install the TubeFixer (Fixer) tweak. Generate a Google API Key: Go to the Google Cloud Console on a computer. Create a new project and enable the YouTube Data API v3 . Create an "API Key" under Credentials. For security and functionality, consider upgrading to a