John Payne, who is a grandfather, has arthritis and walks with a limp, and he wanted to buy the 1.50 pounds slingshot to launch maggots into the water.
Payne, a retired foreman of nearby Bremhill, had at first thought the store assistant was being humorous, but when she insisted on seeing some identification, he realised she was being serious.
He said he was left "utterly humiliated" by the episode in Melksham, Wilts.
"At first I thought the assistant was trying to be humorous. But when I cheekily replied, 'Do I look 18'? she asked if I could prove it - then asked me to verify this with some sort of ID," the Sun quoted him as saying.
"At this point I realised she was serious.
"I'm 73 with grandchildren probably older than her. I understand the girl had a job to do, but she should show discretion," he added.
A store spokesman insisted the assistant was joking.
"I'm sure she was having a bit of fun," he said.