Ned's age is a complicated situation because of the realism about medieval societies in the books. TLDR: every single character is much younger there.
Ned was perhaps 18 by the time he married Catelyn, so he's 34-35 at the time of his beheading. His eldest son, Robb, was about 15 when he had to start a war; Jon is about the same age, Sansa is about 13-14, Arya 10 or so, and Bran is perhaps 8. Dany is about 14 when she's sold off and raped by Drogo. Ser Jorah is 50+, which really counts as old and he's bald too.
I don't recall the age of the other characters, but the generational aspect is sort of there, similar to Harry Potter; so Robert, Cersei, Jaime etc. are about Ned's age, and Tywin is ~50 too.
Obviously, a lot of these ages would be inappropriate even for a show like GoT - child girls getting forced into marriages is a definite no go. But making the girls older also meant aging the boys
and the parents too; and of course the child actors were growing up through the seasons, but I don't mind that. And in the case of Ned, casting Sean Bean makes him look even older (although Bean is still in quite good shape for his age).
Tommen, I think, was quite likely the best of the three Lannister kids, with a pure heart, but his naivety has pretty much determined his fate. It was a sad and still quite unexpected twist IMHO. And I think it devastated Cersei, stripping her of the last remaining pieces of humanity.
The apparent predictability of the show is a temporary thing though. Both Martin and the showrunners are authors interested in good drama, and I'd never expect them to settle for a proper happy ending.
Bringing the plot threads together again required the settling of the 'smaller' conflicts, in order to move the pieces on the board into position for the final act.
Edit: it was also sort of logical to chose which characters should be killed of. The characters with no hope for redemption were the obvious ones, as they had very little interesting stories left in them. This also happened to coincide with most of the 'good' ones; but more on this later.
It is also quite clear that Cersei is not the main villain, but merely one of the final obstacles to remove from the picture before the finale can begin - which also has to include the redemption of Jaime. I'd also say that Littlefinger is not the main villain either, even though he is the person who actually set the current events in motion, by organizing the poisoning of Jon Aryn through Lisa. Lastly, even the Night King is not a villain, possibly not even a character - but more like a force of nature (the showrunners compared him to Death). I do expect most of these characters to be killed off early next season though; unless some of them are better served with the bitterness of living to see the resolution of the story (like, banishing Petyr back to the Fingers).
But the fun part is that the remaining 'good' characters are definitely not going to end up in an alliance. A lot of them are actually being set up with opposing motivations and interests for a start.
More importantly, the main underlying conflict of the story is IMHO the opposition of magic and normality. It has been strongly hinted that the abnormal nature of seasons is directly tied to the White Walkers; and their re-emergence is tied to the return of magic, kinda personified in Dany's dragons. So, my expectation is that the only way to defeat the Walkers is to completely banish any and all kinds of magic from the world - and the price for that is obvious. In fact, if you think a bit about it, the dragons are just as bad as the Night King, they create a balance of power based on terror and destruction. Then there is this Lord of Light, and as Davos has said, if this entity requires child sacrifices, then it has to be evil; and I have a strong suspicion that R'hllor was actually the thing that the sorcerer castrated Varys for (which would also neatly explain how that Red Priestess was able to guess so much about what happened). One also has to wonder if Jon can live without the presence of magic... and let's not forget about the magic of the Children and Bran either.
So, all these elements suggest to me that the endgame is going to be full of twists and tragic moments. It might not be that unexpected though, based on these subtle hints
but it'd definitely fit the style of Martin and the spirit of the show and the books.
