Almost half the 66,000 driver licences reported lost in New Zealand during 2010-11 belonged to drivers aged 18-24, NZ Transport Agency figures show.
Those aged 20-24 were the biggest losers of licences (18,081), and also of stolen licences (4833) over the same period.
University of Otago psychology lecturer Dr Tamar Murachver said those aged 18-24 were not more forgetful than other age groups, but were undergoing "a lot of changes".
"It is a lot easier to remember things when you have very clear routine and stability."
Young adults were more likely to change their accommodation, friends and activities, and those factors, coupled with greater independence, were all possible explanations for the rise in lost and stolen licences, she said.
"When people have more responsibilities, it forces them to be more responsible."
Misplacing or having difficulty in recalling where they left an item were less about the physical changes of their brain than about the transitional nature of their lifestyle.
The age group least likely to misplace a licence were the more than 76,000 licence-holders aged over 80.
A New Zealand Transport Agency spokesman said licences should be treated with the same care as other documents containing personal information. People who had lost their licence or had it stolen should get it replaced immediately or run the risk of identity fraud.
