My point was you're trying to put too much of a real world logistics on the scenario in the movie. As I stated because Talia was born in the pit, her conditions weren't the most ideal to develop properly. Take children in Africa for example. I'm sure there are many boys and girls who are between 12-14 that are underdeveloped by the conditions they live in and don't look their age. Just because the actress apeared to be 8-9 doesn't necessarilly mean it's an accurate representation of the age the character is suppsed to be. Look at Andrew Garfield almost 30 playing a 16-17 y.o. high school peter parker. Take the actor aspect out and let the comic book aspect in.
Nolan took many inspirations from a variety of comic book sources and incorporated them in to all the films. Using Bane as a main villain finally gives Batman a physical adversary. Taking Venom away removes a bit of who he is but makes it more real world and not gimmicky. Making Batman older and worn down and pitting him against a younger/stronger foe makes you cheer more for The Batman and justifies the non-existense of Venom.
Remember the LOS came to rescue Bane, meaning all the time Talia grew up in the Pit, Ras spent with the LOS. If you want to say he can't be the leader of LOS w/in 10 years, if Talia was only 6-8 when she escaped he had enough juice as a non leader to use the LOS to exact revenge? I'd put Ras at about 28 when he impregnates his wife, 14 y.o. daughter makes him 42 when they meet, add the 7 years in between Talia's escape and BB makes him 49/50 for When he meets Bruce.
I'm not saying you're wrong as this would be a feasible scenario in another type of movie. But this is Batman, a movie based on a comic book. Comic book elements and reasoning are incorporated. Do me a favor and pick up or d/l The Dark Knight Returns and read through the parts were Batman fights the mutant leader. You'll notice a big similarity between that and the Bane/Batman fights in Rises as well as in some of the dialogue that Alfred says to Bruce about Bane.