When the number of villagers arrives at or is close to the maximum it is sometimes difficult to keep track of individual villagers. I've discovered a few 50- and 60-year-old villagers without any skills that I've missed somehow.
On the other hand I've discovered 14-year-old adepts and 15-year-old masters to whom I've never given any training. In particular, several doctors often appear to be self-taught.
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To err is human; to arr is pirate.