It's not just about risking your job - I volunteer at events where volunteers run bars, they have to sign a waver form which states that if they get someone's age wrong, then they are liable for a stonking fine and / or imprisonment.
I help out in areas where we try to pick out people who look young for their ages, and our SIA licensed colleagues check their ID. We then give them a wrist band to say we're happy with the age they are presenting themselves as. I've been doing this for the best part of a decade, and in all honesty, it's bloody impossible to tell with some people.
I also sit on the other side of the fence, as only last winter, at the tender age of 37 was ID'd for lemsip. Prior to that, I used to get ID'd about once a year for booze. Mostly when I was in my least made up state - if you ignored my boobs, you'd think I was a 12 year old boy. These days, the boobs are pretty obvious!! Never for knives etc. I think I'd bought them all before those age restrictions came in!
I always keep my driving licence handy, but I totally understand why people might not have photo ID available - DP & I went to the registrar recently to notify our intention to marry, and the amount of ID they wanted was bonkers. Getting proof that you are a real person is bloody costly - citizen cards didn't exist when I really needed to prove my age from 18, so I've no idea how much they cost.
It does seem that these days if you don't drive, and have no intention to travel abroad, you are a bit fucked if you want to do the simple things in life, such as buy a beer, or a packet of fags. It's so sad that cashiers / bar staff / door staff are unable to use their initiative when it comes to judging age.
I don't know what the solution is, I can see things from both sides.