I realized you could play a parent and child team in Traveller. A 6 term retiree and a 1 term child. No real balance in roles or skills, but who cares as long as the players are having fun?
I mostly agree.
But there are lots of different "balance"
The first being a game with no rules which is complete make believe role playing balanced vs a game that has enough rules that it would take years just to read them. This touches on something I mentioned before; a game mechanic with lots of record keeping for experience/skill advancement which detracts from role playing balanced vs having no mechanic at all. The core rules has something extremely short and simple. Perhaps shifted a bit too far towards one extreme for some.
Another thing I touched on before is the realistic limit that people have (in varying degrees) in retaining and being able to recall a huge amount of information balanced to allow people to still learn new things. It may not be the most accurate game mechanism. I discuss this since it was brought up, but I don't recall the INT/EDU limit being part of the core rules? If so, a page reference would be appreciated.
Like you said, there could be characters with low level skills and they could still have fun. They still have a role. The child while others are adults. The one term character that may have a level 1 or even a level 0 skill that nobody else in the group has or maybe they play the assistant to another character. The point here is that characters have a role.
However, you are quite correct that Traveller is much less about 'balance' between the characters than most games ... a 1 term ex-Merchant and a 6 term ex-Navy are not really going to be 'balanced' in the traditional "3rd level party of adventurers" sense.
Perhaps "Game mechanic to preserve imbalance" instead of "Game balance"? Read on.
What if characters in the group had very similar skills as do the groups npcs and most anyone they encounter? I'm not sure if most people would enjoy that. Perhaps the "balance" is a game mechanic which helps limit learning skills and promotes characters being more unique and promoting each having a balanced role vs easily learning what everyone else knows.
There is also the concept of balancing between different aged characters and thus how fast or slow skills are learned. Balance between allowing older characters to be more talented without being so much more so that there is little use for the less skilled younger characters.
And thus we come to solutions that are likely not balanced the way 99% of the people would like but hopefully it is somewhere in the middle and at least people can live with it, ignore it (always an option), or use it to help guide their own system.
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Possible situation
In game, a character gets word that their uncle, a farmer on a nearby low law level agricultural world, was attacked and injured, a few employees killed, and his farm equipment stolen by a competitor. The group goes to help out. The adventure is dealing with the competitor and getting the farm equipment back but the group also stays a while to help out with the farm.
1) Pull out each characters sheet, check what their current skill level is in animals, document that you have x weeks of training and see if they gain a level before they are ready to leave.
Player 1: "Hey, we need to stay another week so that I can raise from Animals 0 to level 1."
This, to me, is playing the rule and not role playing.
2) Fast forward. GM to players: For your time on the farm you pick up Animals 0.
Player1: "Can I do some repairs around the farm and gain Mechanic 0?"
3) Skim quickly over role playing the time on the farm.
GM playing the uncle in his bed weakly telling his nephew
"Go with John and hire new farm hands to replace those killed. It would be a great help if the others in your group can pitch in and divide up the chores."
Character 1:
"I wouldn't know a pigs butt from their face so maybe I should drive the tractor." OOC: has no Animal skill but has Drive (Wheeled) 1
Character 2:
"There were lots of farms where I grew up. I'll help with the livestock." OOC: Comes from a Agricultural World which gave a Background skill of Animals 0.
Character 3:
"Unless you have varmint to kill, someone is going to have to show me what to do." OOC: ex marine and doesn't have skills useful on a farm.
As the GM rules that the characters will only be shown the most basic skills since there are still some farmhands to do the more skilled labor, Character 1 and Character 2 would not be learning anything. Character 3 could get some level 0 training.
Role playing using the skills they have to help out on the farm instead of playing the rules and trying to "level up".
Since it's there, I might check the rules for some guidance. Did Character 3
a) Get some instruction but mostly did general labor. OOC: not enough time available to gain Animal skill
or
b) Learn enough to be helpful with the basic chores. OOC: enough time available to gain Animal skill
For myself and my face to face group we prefer # 3) and just doing what makes sense based on the situation. While there may be some time between adventures, the group is still usually doing something and it isn't down time for going to med school and gain Medic 2 or whatnot.