Limit YouTube to Wi-Fi only
Don't know of any YouTube option for that end, but if you are open to use an app to limit other apps, you can give a go to Onavo Count | Monitor Data that has an option to limit the app to Wi-Fi only:


  Save money on your mobile bill - avoid costly overage fees! Onavo Count puts you in control of your 3G data plan. Easy setup, three brand new widgets, tailored alerts and automatic blocking tools will keep you safe from bloated data bills.  


Update September 2016:
It looks like Onavo Count is no longer compatible with some modern Android devices including devices like Nexus 6P. Onavo has a new "Protect Free VPN+Data Manager", which still has the above functionality in addition to some cool VPN features.