Personally I think it should be increased to 21, bare minimum! There is so many accidents due to children (yes, children) driving too fast. You don't automatically become responsible & able to drive at 17, the same way you don't automatically become an adult when you are 18.
I'm 23 & only just recently passed my test. I waited mainly because I knew I wasn't ready to drive when I was 18, I knew my level of concentration wouldn't be good enough. In the three weeks since I've passed I've seen so much bad driving, a crash between two minors (one was applying lipstick at a red light, didn't see & crashed into the driver at front, who was on his mobile phone in one hand & was eating an ice cream in the other hand, so didn't see the lights change! It's a complete recipie for disaster.. teenagers + mobile phones + loud music + drink/drugs = bad. Not every teenager is an irresponsible driver but more often than not they are & that's why insurance premiums are so high.
Lowering the age will only make it worse.
Now personally I have friends in the car + music but I keep it at a reasonable level & will ask that my passenger/s turn it down if I find it is getting in the way of concentration. I also turn my mobile phone off before I get in the car & will only turn it on & use it as soon as I have arrived safely.