Of course bar staff have to be careful. They're breaking the law if they serve alcohol to anyone under 18.
But life can get very complicated for an 18-year-old who looks 12, or a 13-year-old who looks 25.
My middle son shot up early. He was six foot at 14. Ticket inspectors always assumed he was lying about his age. On London underground, if they saw he'd bought a child fare, they'd chase after him through the barriers shouting at him to stop.
Admittedly, because we're a family of five and always broke, we have sometimes stretched the truth a little. If you're buying tickets for some group activity, and your offspring are classed as adults once they're 14, you might conveniently forget that your eldest has just had his 15th birthday.
"How many child tickets?" says the man in the ticket office.
"What age to you have to be for a child ticket?" you say casually, frantically gesturing at everyone to bend their knees so they look shorter.
Sometimes, being vague won't do. Sometimes your teenager has to produce ID.
But the problem with ID is that it has to be the right kind of ID. A random photo card won't do. A provisional driving licence is good, but you can't get one until you're 17.
My eldest took to carrying his passport around with him everywhere. It seemed like overkill. But he reckoned it saved time.
My friend's daughter recently turned 18. On her birthday, on the way to a party, she called in at an off licence to buy beer. It was a big moment. Her best friend was on hand to take a photo on her phone.
"Would you like to see my ID?" she said to the shopkeeper.
"No, that's fine," he said.
She stared at him. "Please?" she said.
Now it was his turn to look puzzled. "Why?" he said.
"It's my 18th birthday," she said. "Today."
After years of being asked for ID when she'd forgotten it, or didn't have it, or didn't know she needed it, today she'd come prepared. And she wanted her moment of glory.
So the three of them set up a staged photo. The shopkeeper wagged his finger at her while she triumphantly flourished her ID.
"Thank you," she said.
"Happy birthday," he said.
And he gave her two free beers to celebrate.