I just personally don't see that as true with regard to age. There's no reason it needs to be predetermined, just as there's not necessarily a need for appearance to be predetermined. A character, story, and game can be exceptionally well-written without having a completely predetermined protagonist, with the intent being to give players the ability to further immerse themselves in the experience.
They CAN be, but they're a great deal more difficult. Children, teenagers, young adults, middle-aged people, and the elderly all occupy different places in society, and other people treat them differently depending on their age. Same with gender. It's exceedingly difficult to write a compelling story where the character has no distinguishing characteristics at all (except for being human). Harry Potter wouldn't work if he and his friends weren't teenagers.
Psycho wouldn't work if the characters weren't old enough to have relationships but not too old as to look like they've been divorced.
Batman Beyond is introduced by showing how impractical Bruce Wayne is as Batman when he grows old. Archie Bunker would be less funny and more disturbing if he
wasn't old. Could you imagine
The Goonies if it were about a bunch of men and women in their 40s finding a treasure map and all running off to go looking for it? Or
CSI if the forensics team consists solely of 8-year-olds?
The only stories where it's truly irrelevant as to the character's age and sex, and any details wouldn't make for a better story, are ones with settings far removed from our own. Stuff like the Elder Scrolls series, the Mass Effect series, and
Xenoblade Chronicles X work because their settings are nothing like our own, and any type of person could theoretically embark on the story the protagonists of these stories go through. The Pokémon series is NOT one of those. It has fantastic creatures and an infrastructure built around them, but otherwise, the social structure, technology level, and human culture seems pretty similar to our own. The protagonists have consistently been between early tween and late teenagers. That's the age range when they're trusted to go outside and not get in trouble but not old enough that adults take them that seriously.
There are tons of console games that have done it, and the 3DS has the power that some older consoles had when they began featuring games with the option. It really isn't all that much work if the story is written with the feature in mind.
And that's that underestimation that I mentioned. You can't brush off any facet of a major, complex project as "isn't all that much work." Other games have done it, but they haven't told you how much or how little work goes into it. You never can tell what will give the crew tremendous trouble.
For instance, Pixar spent a year trying to get Violet's hair to move right in
The Incredibles. (No 3-D CGI character had ever had long hair in the form of individual strands before.) The scene in the first Muppets movie where Gonzo goes flying into the air holding onto a bunch of balloons was extremely difficult to do. The entire development time behind
Indivisible has had at least one person trying to make believable-looking shadows. It took Steve Oedekirk about 200 kung fu and wuxia movies until he found the one good enough to be used for
Kung Pow! Enter the Fist. The pinball machine for
The Hobbit went through 2 years' worth of revisions to the artwork until they got to one that got Time Warner's approval. These are all things the consumer takes for granted and doesn't look like much work but actually were.
In fact, the reason why Pokémon games used to have such a long lead time to be translated into English is because it takes that long to translate everything, then rewrite all the dialogue so they sound natural. They can do simultaneous or near-simultaneous releases now because each language team collaborates closely with the Japanese Game Freak team, but before that, even the Generation I Pokémon games needed at least several months before a non-Japanese language is ready.