The first thing I'm going to ask, is are the rules of "Legal Age" different in Australia? You said you couldn't drink, smoke, vote or gamble.
In America, you can smoke when you're 18 years old. Everything else has to wait until you're 21 years old. I have agreements and disagreements with the legal age laws. First, my Disagreements.
Disagreements: I find it honestly stupid that you can't gamble (buy a lottery ticket, or a scratch off) at the age of 18.. yet you can smoke. I don't understand why the lottery industry is so strict, you MUST be 21 in order to play. Is winning or losing a ton of money so important, that you can not be a teenager still?
Smoking at the age of 18, is somewhat insane as it is. Children will drink and smoke under the legal age because if for no other reason, thats how it always begins. Breaking the law is "cool" so thats where it starts. Smoking is so much harder to control than drinking, merely because you can't get impaired (minus pot) off of smoking. So short of smelling it on their breath and searching them for cigarettes.. you'll never know.
I think the legal age for smoking should be dropped to 15, if not taken away all together. Now before I get killed by a mob.. I DO NOT smoke.. furthermore, I find smoking utterly pointless and downright stupid. All it does is ruin your lungs, and the lungs of others around you. If I had my way about it, I'd have smokers arrested merely for conspiracy to commit murder. (yes, seriously)
But the fact is, there isn't anything illegal about smoking from 18 on.. so why make it illegal at all? Its going to hurt you at 18 the same it would at 15. Its going to possibly give you cancer at 21 the same it could at 16. So why bother with age restrictions? Other than the fact that once you turn 18.. you have to start following more laws, and taxes. And that would likely be the whole point behind it. Taxes.
Agreements: Now, while smoking (to me) shouldn't have an age restriction (short of 15, anyways) drinking should. In England, or some other countries, apparently you can drink at the age of 16. This to me is absurd and downright ******ed. Why? Because you're still growing at that age, and alcohol can effect you.
Alcohol impairs your system and ability to function. That tied in with the fact that teenagers already make stupid decisions makes you a prime target for rape, murder, and other crimes. Furthermore, I wouldn't hold it against anyone to do any of those things to you if you allowed yourself to stupidly get drunk at such a young age.
Drinking, in America, is only seen as "cool" when its illegal. Those of us who continue to drink beyond the age of 21 do it because its no longer cool, so much as just "allowed" and "normal" to us. However a lot of us actually stop drinking at the age of 21, because its no longer seen as "edgy" its just turned into a waste of money.
Final Thoughts: I don't drink, because I've never seen it as worth my time. I don't smoke, because if I'm going to attempt killing myself, I'm going to do it in a way that won't cause me a lot of pain. And I've never touched drugs in my life, short of them being given to me, by a doctor.
My one issue has and will always be, gambling. Why? Because its the thrill of winning a ton of money, off spending very little. And before you know it, you've spent over hundreds of dollars, and won $10.00.
All in all.. the age restrictions should be placed at 21, world wide for Alcohol, and either lowered to 15, or removed in general for cigarettes. Lottery tickets should be allowed to be purchased at 18. Unless someone can explain why 21 is more magical.