While men believe being young stops at 38 and a half, women are in no hurry to age as the cut-off for them is delayed until 42 years, nine months and three months, the poll found.
Britons surveyed also said that old age begins when people reach the age of 59 years, two months and two weeks, the 'Daily Mail' reported.
The poll, for the Department for Work and Pensions, spoke to 2,171 people aged 16 or over and uncovered stark variations in the views of men and women and different age groups.
Old age, according to women, begins at 60 years, four months and two weeks, but for men, it is much earlier at 58.
The gap may be down to men placing more emphasis on diminishing strength as a mark of ageing, or because women tend to live longer.
Under-50s said old age begins at 46 years and nine months. For those age 50-plus, old age starts at 62 years, seven months and two weeks.
More From This Section
While 16- to 24-year-olds said being young ends at 32, for those aged 80-plus the answer was 52.
This is not far short of when the youngest group said old age begins at 54.
The Department for Work and Pensions said "the disparity in perceptions" showed "the potential for age stereotypes to be applied in very inconsistent ways".
Class and income may also be factors. The unemployed said being young ends nine years earlier than those with full time jobs.
People who lived in council houses tended to believe old age started five years earlier than those who owned their home outright, the poll found.
"These surveys show that the older you get, the later you think old age begins. What is happening is that people are suddenly getting to an age and thinking: no, I'm not old actually. I might have thought I would feel old at this age, but I do not," Ros Altmann of over-50s group Saga said.
"This survey also shows there is significant age discrimination and outdated attitudes among younger people