Psychologists tested 134 volunteers aged 22 to 66 who watched video footage of a mugging in which two men scuffled over a bag.
Participants were then asked to identify suspects from two different video line-ups, only one of which included the mugger.
In both cases, they were told the suspect might or might not be present.
The results, presented at the British Psychological Society's annual meeting in Birmingham, showed that older witnesses were more likely to identify someone who was innocent.
Lead researcher Helen Kaye, from the Open University, said: "It's interesting that older people felt more confident about their selection when they were wrong, to the point where they had imagined someone to be in the line-up who wasn't there.
"As we age, our accuracy as an eye-witness changes."