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I'll change the want to just having the parents visible on a villager until they die rather than until they grow up, so you can identify siblings etc. for longer.

Even then I suspect that would be too much data to store in memory. I'm not a programmer but I imagine the age 18 limit is there for that reason. Maybe it might be possible if the number of villagers were to be cut dramatically, to (say) 50, or their life expectancy were reduced, but both ways would be very unpopular with many gamers.
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