If a child is born with even the tiniest bit of innate skill, they will continue to practice that skill once they reach working age - 14 years old - even if you don't set a preferred skill for them. Of course, that will only happen if they have something to work on in that profession. I've seen 14-year-old scientists actually working at the research table, even though all of the Village Tech was at level 3 and the adults had abandoned the research table. Youngsters with building skill will work on any building projects that are available, which is typically only the occasional debris event that late in the game. If a child is born with no skill, you will have to train them in something or they will just sit around, pace back and forth in village center, or complain about things!
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Barbara
Unicorn
Last Day of Work