Age does not matter, in real life it happens more often than you might think that people even older than that have a regression to their adolescence (adulescent as
Catherine1972 said) and they can act embarrassingly irrational, even more than Sophia.
I think we already discussed about what (could have) triggered her behaviour, like the pre-mature wedding and the need of "growing up" to care about the family and children, missing out other experiences, the lack of attention from the husband etc..
We can discuss if in the game her behaviour changes too suddenly or it is confusing at time, but in general and personally I don't find it "unbelievable"