Your story of being 10 is pretty irrelevant, though, you're just one person. If everybody behaved a particular way in all aspects of life, we'd have no need for any laws at all. The fact that you were useful buying them and not smoking them doesn't mean other children won't do the opposite. These sort of laws have to protect the vulnerable, not build on the basis that all children are grown up enough to look after themselves.
As to drinking in bars, I'm not so sure. In my experience, I have, my friends have and random people I see any time I go out have been able to get very drunk without being thrown out. The only time I've seen people thrown out "for being too drunk" is actually when their being drunk has made them rude and/or violent, often towards bouncers. No-one I personally know has ever got so drunk that something bad happened, but I've seen no evidence that it's harder for that to be the case in a licensed bar than a field. Regardless, even if kids could drink legally, what's stopping them from, after being kicked out for being too drunk, heading to a field with a bottle of vodka?