Mr. Martin’s original intention was that there would have been a five or six year gap between the events of book 3 (A Storm of Swords) and book 4 (A Feast for Crows). However, during the initial writing of A Feast for Crows, Mr. Martin realized that it would not work as intended, and thus he scrapped his plans for a gap after a year of attempting to make it work. The problem was that he found it impossible to jump that span of years without too many flashbacks or expository text, which would break up the narrative too much and leave readers unsatisfied with the result.
What he has now determined is that the events he planned to happen after the five year gap will largely still take place as planned, but will be happening without having to cover those proposed five years. This does mean that some characters will be doing things at an earlier age than he supposed (meaning the children—Bran, Arya, and Sansa—in particular), but it’s an unavoidable problem. He has previously remarked that in hindsight, he should have made the children two or three years older so as to avoid that problem.