It's in their third year that toddlers are able to focus at will, to ignore distractions, and inhibit their impulses. Thereafter, young children begin making choiceful decisions, applying willpower demonstrating self-discipline. By the age of eight, most children have mastery on their mental attention which allows the development of brain pathways for cognitive skills like reading and writing.
Link to the short article here:
http://www.huffingtonpost.com/dan-goleman/-how-children-learn-self_b_4303423.html