That's really shit man. Sounds like he has a real personal issue with marijuana probably the reason he gave you a caution in the first place. 7 million people have smoked marijuana in Australia alone, and its a ludacris to think one third of our populations lives are ruined which is what the officer said.
You can try to reason with them and show them statistics, it may help it may not. I've smoked weed hundreds of times, probably coming up to a thousand and yet I work two jobs and am half way through an engineering course, the truth is this isn't uncommon. Most people I know smoke weed or have smoked weed and very few has it become a problem for (none of which it has ruined there life). The idea that police booklets are the most credible source is laughable. Not saying that their isn't real risks that are no doubt mentioned but these are minimal anecdotal cases. Independent medical journals are the best source of information, as they have no exterior motive other then to display what the evidence shows.
If they keep on your case too much you could try to get them to watch some documentarys on the subject but it's likely these will just come off as preachy when what your parents probably want you to do is to quit using cannabis. What are your aims from here? Weather you quit smoking or not isn't really their business anymore as you're an adult now but that won't change the fact that they're still your parents. If you're still in school the best thing you can do is focus on your studies, if you don't work get a job. Proving that you're productive gives you leverage for you to take you're own risks because if you're a contributing member of society then how you choose to spend your spare time is entirely up to you.
On the bright side, at least it's out in the open now, it saves you the trouble of worrying about a conversation you wish to avoid, it's over now and you can put it behind you. Further more the fact that you got done as a minor is better in the long run then getting a caution now as an adult.