"Jak starý je ten student, s nímž čekáš dítě?"

Translation: How old is the student with whom you are expecting a child?

[ChristophS49077]

Could you also say something like "Kolik mu je, s ..." ? Are there any rules when i can / have to use "Kolik ..." and when i can / have to use "Jak starý ..."?

[endless_sleeper]

You can use 'Kolik je tomu studentovi, s nímž čekáš dítě?'

[ChristophS49077]

Thank you, but could I ask someone about his age with "Jak jsi/jste starý/á?" as well? Because I got taught to use "Kolik ti/vám je?"

[endless_sleeper]

You would be understood but 'kolik ti/vám je?' is much better.