This sounds like a fuss over nothing. The point of the challenge 25 scheme is that it is ridiculously difficult to judge a persons age. Just because a person looks 18, doesn't mean they are!! Retailers face prosecution and losing their licence for selling to under age people, and in NI at least, the person selling the alcohol, and not just the retailer can be fined thousands. By setting the benchmark for requesting ID for anyone who looks under 25, the retailer is protected, the sales staff are protected, and more importantly children are being protected from the dangers of under-age drinking.
Is it really that much hassle to produce a driving licence/passport etc to buy alcohol? I'm 33, so don't get IDed too often, but I take it as a compliment when I do, and then have a laugh with the sales person when they see how old I really am!!
It does sound like Aldi could handle the policy better, if they're challenging people who look under 25, but without bothering to follow it up by asking for ID or applying it consistently, but as far as I'm concerned it's essential to stop underage sales.