Children who miss proper breakfasts in favour of sugary snacks end up with the reaction times of pensioners, say researchers.
Cereals contain more complex carbohydrates
Research carried out at Reading University found that nine to 16-year-olds performed better at mental tasks after a traditional breakfast.
Those who took the tests a few hours after eating fizzy drinks or chocolate reacted at the level of a 70-year-old.
There is concern that childrens' diets may affect their schoolwork.
Conventional breakfasts such as cereals or toast have more "complex carbohydrates", while snacks tend to have more "simple carbohydrates" such as sugars.
These give an initial energy boost, but complex carbohydrates can release energy over a longer period.
The study, published in the journal Appetite, was funded by a cereal manufacturer, but carried out by researcher Dr Claire Pincock at Reading.
She said: "Everyone's cognitive function declines during the morning but we found that the rate of decline among kids who ate a complex carbohydrate rich breakfast was only half of that seen among those who ate a simple carbohydrate breakfast."
The tests involved 30 children aged nine to 16, who were given different foods to eat for breakfast, or no breakfast at all.
Over the morning, they were assessed four times for attention and memory skills.
Those who had the "simple carbohydrate" snack breakfast gave levels of performance that, in other studies, have been given by 70-year-old subjects.
Many more children tend to skip breakfast and fill up on energy drinks or chocolate instead, surveys have shown.
A survey of nine to 16-year-olds found that the average child skipped breakfast entirely 17 mornings out of 100 - and often relied on snack food on other occasions.
Other studies suggest that children who skip breakfast may gain weight because they end up eating more calories in snack food because of morning hunger pangs.