To add more of my $0.02 in...
[personal rant] Thoughout my RPG career I have worked under one primary rule: The subject of balance is not the job of the developers. This is mainly due to the fact that I don't treat these like games I treat them like tools for telling cool stories. The rules are their to create a system of expected "laws of nature" in the context of the world and are not there to give everyone an equal chance of "winning." The reason is simple: Monopoly has to be balanced in order to give everyone an equal chance of winning. If the goal is not to "win," why bother with balance? Any campaign or session that degrades because of complaints of any lack of balance is not the fault of the game but of the group involved. They should reevaluate why they're doing this and, if they can't come to terms with lack of balance in the system, should go back to playing Monopoly. The endless (and absolute pointless) quest for balance, and the expectation that game companies must finalize and approve of that balance, has ruined so many RPGs in subsiquent editions that I can't even care to count them. If the players want to "break" a system and find loop holes, let them. They're going to do it no matter how hard you try to stop them. Don't design Role Playing
Games....design Role Playing
Systems. [/personal rant]
Okay. I've said my piece on balance. Please don't let that rant disuade you from reading the relavent parts below. Now onto the subject at hand.
I see differing cultures to be only effective if truly separated by a genuine "rift" in cultural development. So, in the case of the two mongols, they would both have the same adolescent development (basically the same education crammed involuntarily down their throats) and would have spent later years (the years in which they actually had a choice) developing their own paths. So, really, the difficulties of developing non-socially acceptable skills is still there, but they would have also learned what would be inevitabely learned. And we're talking about two or three ranks here. For the most part this cultural impression would breed familiarity (knocking out that -25) and in only extreme cases (like if they practially grew up on horseback) would they get anything more than, say, 3 or 4 ranks.
So, no, two people growing up in the same town but different households, probably wouldn't develop all that differently in adolescent years. Sure, one might milk cows and the other might tend the wheat fields, but, again, we're talking about subtle familiarities. Not focused training.
I'm honestly not arguing to any point more than another, though, because I also see the concept of "profession" as being an abstract way of saying "cognitive apptitudes. The idea that "profession" corelates to a "job" is a relic of RPGs gone past. Instead I see it as simple way of saying "what that character is born capable of learning faster or slower due to natural talents and apptitudes."
Of course, one could argue that this is what Attributes are for. Attributes translate into natural talent. You have higher attributes that lend bonuses to certain skills which leaves your
culture to decide what forms of training you have been familiarized with (i.e. what skills in which you have developed a rank to remove that -25 of total unfamiliarity).
Both methods I see as having merit, but the use of the "fixed" adolescent costs just makes more sense to me. Nature vs Nurture. Certainly not a question to be answered on these boards. But since Attributes fill the role of natural talent, and the choice of profession can represent your focus due to interest (as well as some measure of aptitude), I see Culture as being a third facet that is very valuable in developing each character in yet
another way.
So instead of Talent/Aptitude/Aptitude, you get Talent/Culture/Aptitude. See what I mean?