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 ..."?
You can use 'Kolik je tomu studentovi, s nímž čekáš dítě?'
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?"
You would be understood but 'kolik ti/vám je?' is much better.