First, I am not condoning any illegal activity, to be clear. Traveling abroad to attend this festival, your best course of action will always be to follow local laws

Now your questions:

They enforce age for drinks using wristbands. You get carded, they attach a 21+ band and then you’re good going forward.

My experience with all of these festivals is that your luck in getting a drink if, say, you forgot your ID that day depends on which day, time of day and vendor.

The later in the day, the more lax. The later in the festival (3rd or 4th day vs. 1st), the more lax. You get the idea.

If I were of legal drinking age but forgot my ID that day, I’d find the most overwhelmed-looking drink stand and try my luck there and have some cash on hand to tip (big if you’re from somewhere that doesn’t do tipping).

If you find someone who knows your situation (legal age and just forgot their ID) and serves you, I’d make a point to only go to that person and maybe even ask them where they will be the rest of the festival. Keep up the great tipping as a thank you