This probably varies depending on where you live, but where I grew up (in a small town in the UK), the vast majority of 13 year olds did have access to drugs, alcohol and tobacco (which didn't necessarily mean the vast majority were indulging in those things, but they were definitely available).
I mean it’s somewhat true, if it were legal for 12 year olds to have these things AND it was marketed toward them, way more kids would be have it than not. That’s not to say they can’t get it now, but it’s harder and somewhat self regulating because the stigma around doing something illegal.
This doesn’t invalidate your point (I think your correct that making it illegal provides a disincentive and reduces overall uptake), but I should note that making something legal does not necessarily mean making it legal to market it towards kids.
Case in point being that here in the UK it is illegal to market tobacco products at all (including to adults in this case). This also applies to politicians, prescriptions medicine and a bunch of other things. And IMO it’s often a good compromise that allows for harm reduction without outright banning something.
