There’s no law against teenagers drinking alcohol, but it can only be sold to those who are over 18. Any adult selling liquor to younger person can be prosecuted. Pubs and clubs can stay open 24 hours.
It is acceptable and sometimes quite common to walk with an uncapped beer in hand in the Czech republic. But in certain areas of Prague is banned to drink in public, especially at Prague 1. If you are caught drinking you may face a fine of 10.000 Kč.