Translation:I cannot drink beer because I am a child.
What's the difference between můžu and mohu? Or are they interchangeable ?
Completely interchangeable, it is a different form of the same verb, but mohu is more formal, may sound bookish in spoken (except very formal) conversation.
Děkuji! mohu is just easier to remember... but I'll keep this in mind when\if I have a chance to use Czech in real life