@The Big Peat I second your recommendation of Lois McMaster Bujold's
The Curse of Chalion.
When I turned thirty years old, I found I had not only lost any taste for teenage heroes, I discovered that I actively sought out stories with characters in their thirties. Now, at fifty-six, I still gravitate towards characters with whom I share similar life experiences. I think this is natural human behavior.... I assume almost all of us are doing it. And yet, in my opinion, there is always an influx of creative arts marketed to the public from younger authors, painters, film makers, et al. and that the target audience of these marketers is a younger audience. We, the posters in this thread, are already die hard and true believers in sci-fi and fantasy literature. We will sift through to find what we want, but the years of our purchasing are less and less. Marketers need to hook audiences with forty years or more of purchasing ahead of them.
@Toby Frost I also am interested the term "fully integrated."
I'd also like to comment upon your reference to
A Game of Thrones. In AGOT, George Martin writes each chapter from the point of view of a character.
Eddard Stark, aged somewhere 33-36
Catelyn Stark, 31-34
Tyrion Lannister, 23-26
Jon Snow, 14-15
Daenerys Targaryen, 13-14
Sansa Stark, 11-12
Arya Stark, 9-10
Brandon Stark, 7-8
SPOILER ALERT
It seems fairly clear early on that Jon and Dany are destined to be the two candidates to lead the next generation. Throw in Sansa, Arya, Bran and their brother, Robb, and you have six strong and noble characters on arcs to lead/save the world. We are also given Theon Greyjoy, 19, Joffrey Baratheon, 12, in the first book. Fast forward through book five,
A Dance with Dragons, and perhaps three or four years have passed and these characters have experienced much more of life. As expected, Sansa is betrothed to the Crown Prince, Joffrey becomes King, Dany gets three dragons and becomes a Queen, Bran gets magical powers, Jon becomes Lord Commander of the Night's Watch, Theon's father becomes a king and makes Theon a warleader, Arya gets trained by a master swordsman, and Robb becomes King in the North.... BUT teenage kings Joffrey and Robb are both assasinated, Bran is crippled and left in a cave for the rest of his life, Arya loses all family and identity, Theon betrays his best friend and gets tortured and gelded, Sansa has to watch her father's murder, Dany's rule crumbles to revolt, invasion, and disease, while Jon is murdered by his own men.
Martin seems to be making the point that teenagers with wolf pets, dragon companions, magical powers, impeccable lineage, beautiful bodies, and fabulous riches will all amount to nothing because of foolish decisions stemming from a lack of experience. Teenagers thrust into world leadership don't save the world, it burns.
On the other hand, Eddard and Catelyn, the experienced adults also get themselves betrayed and murdered. Tyrion, a nominal villain, is the person with the healthiest respect for the dangers of the world and a solid grasp of his own strengths and weaknesses.