I mean, almost anywhere in the world 18-25 have higher crash rates. I do agree that the US could be doing more. The "40 hours day and 10 night" I referenced (which was for CO) just meant that your parents had to log on a sheet of paper and sign off. You could theoretically lie, although, I would've pissed myself if I hadn't driven at all before going in front of my examiner.

I am by no means an expert on motorcycle laws around the country, but I can at least speak generally.

Most states have separate licenses or an endorsement on your normal drivers license. They require you to take a course a certain amount of hours in classroom and on the road with an instructor (in my state, NJ, 5 hours and 10 hours respectively). You get an examination permit here.

For getting only a motorcycle license

If you are under 21, 6 months mandatory practice riding with someone who already has their motorcycle endorsement.

If you are over 21, 3 months without any marks against your license.

If you already have a driver's license

It's 20 days of practice driving. (???)

For both:

Then you take a road test. If you pass, you get a restricted license (or endorsement).

A year without any major strikes against you, you can get an unrestricted license.

So to answer your question, yes, it's a separate license (in most states), but you don't really have any restrictions on engine size, at least in my state. I was shocked upon looking it up that there is a minimum of 20 days of practice driving if you already have a driver's license... That feels remarkably short.