Test purchasers are 16/17 YOs who may or may not look 18+. They are not people in their mid 30s who may look 24 in a darkened room.
Actually, some stores (mine included) will send in their own test purchasers, and employ Mystery Shopping Companies to do the same, where they send in people who are over 18 but look under 25 (including some places that will send in 26/27 year olds who look under 25) ... if you don't ID them then you fail your test purchase, even though they are not under age, and you will be subject to your store's disciplinary procedure. If you fail one test then you'll get a written warning on your file ... if you fail another within 12 months you can lose your job.
Then there's the impact on a store as a whole - if you fail 3 test purchases in a row then the store will be put under investigation and will have even more test purchasers sent in than usual. Fail another test purchase in the next 3 months and there are all sorts of serious consequences including staff losing their jobs, the store being fined and loss of licences. And that is on top of the county council and police test purchases where they send in genuinely underage customers to buy things, which risks personal fines and loss of jobs too.
It's silly tbh. The legal age is 18. They should be challenging people who look under 21, under 25 at a push.
That is how the policy is put into practice ... when it was "Think 21" far too many people were slipping through the net though because a 16/17 year old and a 21 year old look far more similar than a 16yr old and a 25 year old, and a lot of late teens - especially girls with lots of make up etc - can easily make themselves look a couple of years older, so they widened the gap. So when a customer appears, we are trained not to think "Do they look over 16/18?" but "Do they look 25?".
The policy unfortunately is not infallible as it relies on the cashier's judgement - I'm 27 so I have a policy of anyone who looks younger than me will get ID'ed ... no ID? No service. I don't care if you're 29 with your child ... your child is not ID any more than your insistence of your age is (Just like my 8yo DD proves nothing beyond the fact I had a child 8 years ago).
I have in the past ID'ed a guy I thought was 25, the store had just failed a test purchase so our managers were cracking down on lax application of the policy, I asked for his ID just to be on the safe side ... turns out he'd only turned 21 a few months previously. Another time I ID'ed a girl I thought was maybe 19 ... she was 25.
I tend to find, as a cashier who works the customer service desk and lottery/cigarette kiosk so deal with ID'ing people multiple times a day, that most people are actually pretty good about having their ID on them, but if they don't have ID I have found that the older a customer claims to be, the more argumentative and demanding they tend to be. Now this is purely from my own experience but if I ask a younger person for ID and they realise they don't have it the exchange usually goes:
"Can I have 20 Sterling Dual please?"
"Sure, can I just see your ID first, please?"
"Shit, I forgot to bring it with me"
"Sorry, I can't serve you without your ID"
"Yeah, no worries. I'll nip home and get it/get my fags later"
However, with a lot of "older" customers the conversation usually ends with them demanding the manager (who, in my store at least, will always 100% back up the colleague when it comes to the Challenge 25 policy):
"Can I have 20 Sterling Dual please?"
"Sure, can I just see your ID first, please?"
"Are you joking?! I'm 27!"
"Sorry, you look under 25 to me, so I have to ID you"
"Well I'm 27, this is a joke! I've got my daughter/son/mum here - mum, tell them I'm 27!"
"I'm 27 as well, I still get ID'ed sometimes so I know it's a pain but now that I have asked to see your ID I can't sell you any cigarettes until I have seen it."
"Fine, I'll just go over there and get the other woman to serve me"
"You won't be able to do that as I will be informing her that you have failed to provide ID when asked and are trying to evade the rules"
"This is awful, you're embarrassing me in front of all these other people, I want to see a manager, this is terrible customer service!"
Occasionally I have even resorted to asking the customer to tell me how old they think I am before telling them my actual age in order to try and get them to understand that it's not always easy to guage how old someone is from a quick glance. I have had answers ranging from 20 (I wish I still looked that young!) to late thirties (rude!) but even the "late thirties" guess shows that someone can easily be 10 years out with a guesstimation of age based on their perception of what age you look. I'd rather risk the earful off pissed off customers than risk my job and if you are the 29 yo mum who unfortunately gets IDed for your wine, sorry but your wine will have to wait until you have your ID on you.