Just use whatever Web browser you have on that old iPad to accesss the actual YouTube Website and log into your YouTube/Google account ID to login to your YouTube content.
You can watch YouTube videos from the iPad mobile Web browser.
If you need an more up to date and more secure Web browser for such an old iPad model look at this third party Web browser, in the iOS App Store, called Alook.
Alook
https://apps.apple.com/us/app/alook-browser-8x-speed/id1261944766
It is, definitely NOT 8X speed on older iPad models, but is faster and more up to date and aecure than the 6-year old vetsion of tnd iOS Safari Web browser.
Best of Luck to You!