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While males can produce children their entire adult lives (from age 18 on), females will be unable to produce children once they reach age 50. They can still "go indoors" after reaching age 50, but the population won't increase as a result. That would explain what you saw happening.
As for the "unauthorized" trip indoors, the villagers will occasionally decide to use any of the skills they possess, even if you have selected another skill as preferred. The more skill they have in a particular area, the more likely they are to use it once in a while, despite staying on task in the preferred skill most of the time. It can be a very risky thing to allow males to gain a lot of skill in parenting, unless you're prepared for a population boom.