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.
Men say being young stops at 38 and a half. For women the cut-off is delayed until 42 years, nine months and three months.
Old age, according to women, begins at 60 years, four months and two weeks.
But for men, it is much earlier – at 58.