I was riding a bike to school on the streets when I was 8. Would he license 8 year olds? As donnamb pointed out, schemes to license cyclists inevitably fail once they meet the details of reality.
On a policy level, the reason we license motorists is that driving a car or truck poorly creates a risk to others. Motorists slaughter thousands (hundreds of thousands?) of people every year. If cyclists start to kill and maim as many people as motorists kill and maim, cyclists should be licensed. Until then, ticket us when we break the law, but otherwise leave us alone.