Answer:
You're asking for an opinion, so here's mine. Personally, I don't think it makes much difference. In my experience, the average 18 year-old is every bit as reckless and irresponsible as the average 16 year-old. At least, as it is now, an 18 year-old - who would be faced with long periods of driving, a daily commute to qand from work, etc will at least have SOME driving experience, even though it doesn't necessarily mean they're prone to drive responsibly.