I would think completely going without carbohydrates at 14 would be just fine. Why does one really need sugar anyway? All it does is make things easier, but isn't necessary.
At 23, being ZC for 3 months now, I would go back to my childhood years and GLADLY change to a diet without carbohydrates. The benefits of optimized human growth hormone during that growing period would have been very interesting, and astounding to experience. A childhood without chronically high insulin I'm betting would have made the pubescent period much easier to stomach. Under 30g a day would do just fine, but the benefits of full fat-adaptation wouldn't be experienced.
And IMO, ZC has great advantages over 30g of sugar a day. A completely clear mind not being drunk off glucose, a stable sense of well-being, sharper intellect. A higher state of existence that I lose with any ingestion of glucose.
Also, per the mentioning of better athletic ability... I attest to this as well. I perform better zc. Sprints, handstands, agility, dexterity.. especially 15 or so hrs fasted in conjunction.