To be honest if I had asked you for ID because I wasn't sure and that was your response, I'd just be grateful that I didn't get a tirade from you with personal insults and making a scene, because that is more common than it ought to be for any interaction between customers and staff, without provoking it for the sake of it.
While there probably are some people who serve alcohol that are like that, I don't particularly like being shouted at, personally insulted and mocked. On the other hand I do quite like having a job in order to pay my bills and staying off the radar of the local police, licencing team and not ending up on a disciplinary.
I'm not trying to take over the world one refused sale at a time. I don't want to be responsible for what you do under the influence of or with the alcohol I've sold you, or if you are purchasing it knowing full well you're under age and it's illegal to do so.
I'm as much at the mercy of these rules as you are, I'd much prefer it to be that people take responsibility for themselves where alcohol is concerned.
The laws and licence conditions are the government paying lip service to the issues that alcohol abuse causes within society and the money it costs from policing, the NHS and councils, instead of actually tackling it properly.
By loading the responsibilities onto the retailers and by extension their staff, and not the people actually buying and using it. And society, ime, backs this when something happens, a young person near me ended up dead because of alcohol not so long ago - one of the first questions asked was where did the alcohol come from, because whoever supplied it was to blame because they were, by a few months, under age.
But the crux of the matter is that people do not like being 'challenged' by someone they see as there to serve them, because they feel then that the balance of 'power' between server and customer is off and that as a customer, they should never be questioned and certainly not refused.
But while most of the concequences fall on the person selling it and the person holding the licence for the premises, very little will change because basically, I'd rather you shout at me, personally insult me and misguidedly think you're important enough to me for me to want to piss you off on purpose, than I face breaking the law, or breaking licencing conditions and any sanctions on the business I work for being imposed.
It shouldn't be a choice between the two, but while people are determined they 'know' licencing laws when they really don't and can't be bothered to do a bit of simple googling (for example mandatory conditions) and have the entitled attitude towards alcohol you see quite often, that is the choice.