What does this mean?
Maybe they've been reading too many stories about the rich and famous. Adele, for example. This year's Sunday Times Rich List reveals that Adele topped the list of wealthy young musicians with assets of £20 million. (Jessie J, worth only £5 million, trailed at joint 14th.)
Maybe if you see people just a few years older netting an absolute fortune, you think it's very likely you'll do the same.
It's also possible that teenagers have no idea what the average wage is. So they pull a figure out of the air. Posh boy David Cameron, according to Tory MP Nadine Dorries, doesn't know the price of a pint of milk. Maybe there are teenagers who have never seen adults fainting at the sight of the gas bill.
But I suspect, on balance, that the real reason that most teenagers think they'll be earning twice the UK average salary by their mid-thirties is that youth is a time of optimism.
The other day, I was having a cup of tea with a friend in the kitchen. She said, "So what do they want to do, then, your kids?"
"I don't think they've decided yet," I said. I sighed deeply. "It's going to be so much harder for them than it was for us. Recession, job cuts, student loans..."
She looked at me.
"What?" I said.
"They're young," she said. "They'll be fine."
And I thought, after she'd gone, that's true. They'll find a way out of this horrible mess. Because they're young and energetic and full of hope. It's what youth is all about really – the certainty that you'll sort it all out much better than your parents did.
It's too late for me. I'm just going to sit in the corner with a cup of tea. Having a good old moan.