Couldn't it be "your woman is young"? It's not particularly polite but you can say that in English in certain social circles.
I am not sure what circles you are moving around but no. Woman in English does not necessarily mean that the two people are married to each other.
It does mean that they are in a relationship, but not necessarily married
How would you say in Czech "your woman is young"?
The same, but it does not make much ssense in English, does it? What do you mean by "your woman"?
If your partner for Scottish dancing is female you could certainly say that your woman is young unless she is your wife.
That is irrelevant for the Czech sentence you were asked to translate.