It makes them look silly and you say 'no problem, I'll take my custom elsewhere'.

Then do prank phonecalls on them ALL NIGHT LONG.

Say you haven't needed to show ID in a while, so rarely carry it these days. But that you have a credit card to show you're over eighteen.

(yeah ok i'm not actually 18 but)
You just make it seem like they're being unreasonable, which in your case they are. First of all you have to pretend you have ID on you. Then you say "Oh come on give me a break for gods sake", then you shake your head and walk out.

i say "oh, i don't drive" with a whole demeanor like i am 21, like it's really surprising i'm getting ID'd. but if you are actually legal, why would you look like a twat? you aren't trying any sneaky business.

and if they don't serve you leave

sniff and say whatever and walk off doing Liamy G's gibbon strut. As you leave knock loads of items off the shelves, then spit at an old lady and shout "You want some!?" Thus avoiding just looking like a twat and actually BEING one

I usually suggested that if I'm under 18, the credit card Im using to pay for said booze wouldn't be much cop would it.

I can't remember the last time I got id'd, probably when I was in Ameica 4 year ago.

i tend to chuckle a bit and then proceed to fan out all forms of ID, saying "pick a card, any card"

Spend the night in a cell.

is my South African Drivers License and I'm always getting asked for ID...they will literally stare at my drivers as if I have faked it.

It's so annoying!