AJ Reeves, 13, won a free £1 McFlurry in a Monopoly promotion after buying a Big Mac.
See also: How to win at McDonald's Monopoly
See also: Happy Meal toys are worth a fortune!
But when he went to collect his prize staff told him he was 'too young' and needed an adult present.
McDonald's say the game is for over-16s - even though there are no age restrictions on buying food items.
AJ, who suffers from Asperger's syndrome, said: "I felt sad and disappointed.
"I'm not sure whether I will go back."
Mum Kay, 52, from Worcester, is writing to McDonald's to complain.
AJ with his winning ticket
His mum said she felt like McDonald's was 'tricking' kids
She said: "I can completely understand if McDonald's want only 16-year-olds and over to play.
"But if that is the case they should not be selling these vouchers to under 16s.
"It feels like they are tricking kids into buying their meals without letting them claim the prizes.
"Monopoly is a child's game, it's clear that's who they are aiming this at."McDonald's said the promotion was only open to those over 16 and younger children could get an older relative to claim prizes
McDonald's said: "In accordance with Ofcom guidelines, and as clearly stated on all McDonald's Monopoly promotional materials, advertising and packaging, the promotion is only open to those aged 16 years old and above.
McDonald's in Blackpole Retail Park Worcester
"Monopoly prize stickers are not included with a Happy Meal and whilst it is possible a person under 16 could purchase a product sold within our promotional packaging, the terms and conditions are clear that they are unable to take part in the promotion.
"Any prizes can be claimed by older family members."