A couple months ago, Martin published "Fire & Blood" a narrative history of the early Targaryen dynasty in "A Song of Ice & Fire" and the people like I am about WoT, except for aSoI&F, but feminists, have been having shitfits. These people would take bullets for GRRM, they passionately defend the Quentyn Martell plotline in aDWD. But the history of the Targaryen dynasty, in addition to incest, is rife with underage marriages. Like junior high ages. 11, 12 & 13 year old brides. And a lot of them die in childbirth, because they are not important to the story and Martin wants to get them out of the way. And the feminists are absolutely livid.
But there is a very simple Doylist reason why Martin keeps doing this. See, when he put out the very first book, A Game of Thrones, he included in one of the appendices, a timeline of the Targaryen kings with dates and their relationships to one another in the succession. So we know the generations of Targaryen kings. But we also have the dates and generations of real world kings. Lets take a look:
Generation 1
Aegon the Conqueror (reign began in 1 AC)
William the Conqueror (reign began in 1066 AD)
Generation 2
Aenys & Maegor (Aegon's sons)
William Rufus & Henry I (William I's sons)
Generation 3
Jaehaerys I (Aenys' son)
Stephen of Blois & Empress Mathilda (Henry I's nephew & daughter)
Generation 4
Baelon (Jaehaerys' son; not named in aGoT)
Henry II (Mathilda's son)
Generation 5
Viserys I (Jaehaerys' grandson, Baelon's son)
Richard I & John (Henry II's sons)
Generation 6
Aegon II & Rhaenyra (Viserys' kids)
Henry III (John's son)
Generation 7
Aegon III & Viserys II (Rhaenyra's sons)
Edward I (Henry III's son)
Generation 8
Daeron I, Baelor (Aegon III's sons) Aegon IV (Viserys II's son)
Edward II (Edward I's son)
Generation 9
Daeron II (Aegon IV's son) {or Aemon the Dragonknight's}
Edward III (Edward II's son) {or William Wallace's}
Generation 10
Aerys I & Maekar (Daeron II's sons)
Prince Edward of Wales, Duke John of Lancaster (Edward III's sons)
Generation 11
Aegon V & Maester Aemon (Maekar's sons)
Richard II & Henry IV (Prince Edward's & Duke John's sons)
Generation 12
Jaehaerys II (Aegon V's son)
Henry V (Henry IV's son)
Generation 13
Aerys II (Jaehaerys II's son) Died in 283 AC, 283 years from the Targaryen Conquest
Henry VI (Henry V's son) Died in 1471 AD, 405 years from the Norman Conquest
George is trying to squeeze 13 generations of a family into less than three centuries, while the real world family which most inspired the politics of his series took over four centuries to beget. As a result, to satisfy all the nit-picky fans, who are his equivalent of the people among us, who flipped through thousands of pages of novels to sort out the Aes Sedai strength hierarchy, Martin wrote a book explaining how those first seven generations came to be and pissed off many of those self same fans, because the only way to make it all fit, would be to offend their sensibilities by making their mothers 12 years old.
Math is important, kids. And sometimes less is more in world-building.
"It just happened" can often be a vastly more palatable explanation for some contrivance of the plot then one that actively draws the audiences eyes to how little sense it actually makes.
