Why should there be age restrictions once you're legally an adult at 18? 21 is unnecessarily puritanical (EU countries don't have that) and most teenagers are going to drink anyway, just as they're going to find a way to bypass age verifications for their social media of choice.
This is essentially the law in the UK. To be able to drive a motorcycle of unrestricted size you need to be 24, or have worked you way up by holding a license for a smaller size class.
The difference is that driving is a privilege not a right (riding a motorcycle especially so), as you are endangering the lives of others.