Kate Lancaster was asked to prove she was over 18 when she picked up some watermelon and grapes as it was believed it could ferment and turn to alcohol.
The 37-year-old from Plymouth was attempting to buy the £1 tub at the self-service till when the requirement flashed up.
But luckily a shop assistant cleared it without asking her age.
Baffled, Kate emailed the store and received a response saying: "All fruit will be age-verified in case natural fermentation takes place."
The library worker was quoted in The Sun saying: "A friend and I fell about laughing. It's incredible. You couldn't make it up."
She also pointed out that the rule could result in children being prevented from getting their five-a-day.
Talking about the situation, Tesco explained: "The comments about fermentation were a joke. We've no plans to age-restrict fruit."
"We've apologised and sent her a fruit basket as a goodwill gesture."
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