Upvote:1
Carry your passport. But keep it safe under your clothes in a special pouch.
When you need to proof your age try your driving license, if it is not accepted you can pull out your passport.
If you have a passportcard you could try that, might still not work.
A paper copy of your passport is very unlikely to be accepted, too easy to fake or change details.
In some countries people have to carry their passports I am not sure about Portugal.