Never. There should never be an age to start any caffeinated products at all.
Preferably never. Although, coke products are not contraindicated or banned for use in children, I would not recommend them to be used by children or adults. Consuming coke products carries no benefit whatsoever to either children or adults and, in fact, may cause long term detrimental consequences such as obesity and diabetes (due to high sugar content), kidney stones, indigestion, caffeine dependence, etc.