I think my own issue with your comments is that you seem to equate "not having the most ideal spells chosen" with... being 8 years old.
Actually, no, that was a response to someone mentioning the play style of eight-year-olds.
This entire mindset of "you don't have any place playing a 12th level character without meeting a certain requirement of system mastery"... that is everything people are complaining about 3.5 in a nutshell.
That's not my position at all. I feel that by 12th level, 100+ encounters should impart a minimal mastery of the system. If you start the game at 12th level, I hope you have experienced players. If not, expect the unexpected. I would not hammer a group of novices playing 12th level characters... I would not
run a game at 12th level for novices, I would pick somewhere in the 1st to 3rd range so they would have some chance of understanding what was on their character sheet.
Why should I have a character with the perfect spells and equipment prepared for every possible encounter?
Never said that. A cold iron morningstar is not the "perfect" weapon for very many characters, but it is 1) blunt, 2) also piercing, 3) wieldable in one or two hands, and 4) cold iron. It's an all-purpose tool, and owning one saves you all kinds of what-if scenarios.
Why shouldn't I instead just be able to build an appropriate character concept who is effective at what he does, and not have this mean my instant death when shadows come pouring out of the walls? Why can't I have a wizard who enjoys hurling fireballs at his foes, rather than driving them away with repulsion and trapping them with walls of force?
Everybody needs a day job.

But seriously, if you are going to throw fireballs, then you should look at Consecrated Spell and Trandimensional Spell, to get you non-fire damage and the ability to affect incorporeal foes, respectively. There are numerous options, but you should gear up for more various situations than just barbecuing orcs.
There shouldn't be a certain list of items or spells required to succeed.
That is correct. But there is also no guarantee that dragons will be nice to you if you do not explore any good adventuring options at all. There is not one specific way to be a trial lawyer, but there are still competent lawyers and less competent lawyers. There is not just one painting style, but every good painter needs to develop a style that works.
There shouldn't be a "you must be this tall to ride" sign, with a certain level of system mastery needed to play at level 12.
I'm sorry, here you're just wrong. It's not the GM who makes 12th level challenging, it's having 12 levels of options and facing CR 12 foes.
And the expectations you seem to have are very high - not just for random 8 year old players, but for plenty of people who have played this game for years.
Maybe. Apart from the "ghoul incident" about three years ago, and one crazy critical hit against a non-raging Barbarian, the only deaths I've seen in my games were immediately proceeded by warnings from other players that the player in question might want to reconsider their actions. This is in a 1st to 18th level campaign. So I think overall my expectations are pretty realistic.
You might be shocked that a 12th level wizard didn't have wall of force, and not know what to say. For myself, I am similarly struck speechless by the idea that you cannot even conceive of someone player a wizard at that level without that one specific spell.
It's a handy spell. I mean, seriously, at 9th level, there are lots of choices, but by 12th level, when you have 6th level spells, I think that's a good 5th level spell to know. If you are a sorcerer, sure, there are always hard choices to make, but for a wizard, getting a 5th level spell means, at most, shelling out some gold or going on a minor quest.
Force wall blocks doorways, halts forward movement, blocks most spells, blocks dragon breath, and is very difficult to get rid of.
I have a hard time imagining a wizard not having magic missile (unless Evocation is a banned school, of course) by 4th level. Same thing. Is there any 12th level wizard who does not know magic missile?