It absolutely was GRRM's idea, and he's stated on his LiveJournal both that he originally started the characters very young because he thought he was going to have a five year timeskip early in the plot (that ended up being unworkable, see also the Meereneese knot) and that if he could change one thing about the books, he would have made most of the characters their show ages from the beginning.
There are definitely differences though! One I've noticed in DWRP (possibly making anon pointless but oh well) is that Theon's relationship with the Starks in the show is very different. In the books, Robb, the eldest, is still a good five years younger than Theon and Theon sees him as a little brother and barely notices the younger siblings. His loyalty is loyalty to the Starks as a house and Robb as a fondness. In the show, they're obviously much closer in age and have experienced various coming of age social rituals together, so therefore Theon's eventual betrayal is more acute. But for me personally, exploring that and making book canon work with show ages was part of the fun.
Re: GAME OF THRONES
There are definitely differences though! One I've noticed in DWRP (possibly making anon pointless but oh well) is that Theon's relationship with the Starks in the show is very different. In the books, Robb, the eldest, is still a good five years younger than Theon and Theon sees him as a little brother and barely notices the younger siblings. His loyalty is loyalty to the Starks as a house and Robb as a fondness. In the show, they're obviously much closer in age and have experienced various coming of age social rituals together, so therefore Theon's eventual betrayal is more acute. But for me personally, exploring that and making book canon work with show ages was part of the fun.