Someone upthread said that 17 is too young for a first-time rider, and that one should either start as a child or else as an adult.
As I said, I started riding when I was five. At 12 I was riding a bike that was too big for me, but I outgrew it in a few years. I believe that years of off-road riding made me a better street rider. In the dirt you have to watch out for soft sand, rocks, bushes, and other obstacles. It hones the reflexes. And even after not riding for a few years after a couple of non-motorcycle injuries and getting a car the reflexes were still there. I was 17 once. But I honestly can’t remember being reckless on my bike. I was a little more responsible than a lot of kids, but I think it was the years of riding that kept me safe. My Enduro wasn’t a ‘new toy’. It was something I grew up with. (Well, not the same bike; but you know what I mean.)
I think you all know I love motorcycles. I’d like to see more people riding them for a number of reasons. But having been 17 once and being a lot older now, and after reading this thread, I too will have to recommend caution.
My Yamaha 250 Enduro could manage 65 mph on the road. (It could go a little faster, but it wasn’t designed for it.) I was quite used to riding the 100 before it, but even so the 250 was not a performance bike. They didn’t have sportbikes in the '70s. Now it’s very easy for anyone to go out and get a street-legal racing machine. Fast bike + 17-year-old = Trouble. But still, motorcycles are fun, economical, and fun.
Obviously you can’t go back in time and start the kid riding at ten. And it would be irresponsible for him to get a cool racer. Where’s the compromise? One option would be a '70s-vintage small standard such as a Honda CB350 or CB360. They have two wheels (the most important criterion), they’re cheap, and they aren’t spectacular performers. But they lack the ‘cool factor’. Of course you can always spin it as a ‘retro bike you can’t just go out and buy in a shop’. Old and slow? No, a classic! 
But a lot of kids won’t see it that way. So how about a Kawasaki 250cc Ninja? It has the looks, and it’s not like it’s a YZF-R1. But it will still ‘do the ton’. You know he’ll do it just so he can say he did. I can tell you from experience that Bad Things can happen when you get near the edge of the performance envelope, and that it takes experience to get away from them. Ask me about high-speed shimmy. And even experience mightn’t help.
Still, a 250cc bike isn’t too bad a choice for a first ride for a teenager. It comes down to this: How mature is this person? Is he going to show off just because he’s Mr. Cool Motorcyclist? Will he try to do things he ought not to when his peers on larger, faster bikes belittle his ride? Does he have the self-control to learn how to ride properly and not go beyond his limits? (It can be done!) A 17-year-old is going to pull some boners. But a responsible 17-year-old will make an effort not to get too far out of his depth.
They didn’t have motorcycle classes when I learned to ride. If I had a kid I’d start him or her out early on dirt bikes. But if I suddenly had a 17-year-old I’d insist on a low-performance bike, training classes, a full suit with armor, and a stipulation that any transgressions will result in the loss of riding privileges.
I hate to see anyone not being able to ride. But some people shouldn’t. It all depends on the maturity of the rider.
Padded ski jacket and a helmet.
