By Jonathan Amos
BBC News science reporter, Dublin
Get to 49 and it seems you could be "over the hill" - at least that's how many of us perceive it.
Forty-nine is the age that people, on average, in a Kent University study stated that youth came to an end.
The UK-wide survey has thrown up some interesting facts on how we view people who are older or younger than us, and just how far our prejudice extends.
And it is clear that teenagers as well as pensioners can sometimes feel put down because of their age.
"We shouldn't forget that one of the important targets of ageism is young people. They feel very aggrieved about the stereotypes that portray them as nasty yobs who are drunk all the time," Dominic Abrams, a professor of social psychology at Kent, said.
His study was conducted for the charity Age Concern. Some of the results have been released here at the British Association's Festival of Science.
They come from detailed interviews with 1,843 people over the age of 16, and they appear to show that age prejudice is ubiquitous in British society.
More people (29%) reported suffering age discrimination than any other form of discrimination.
"Ageism is the most pervasive form of prejudice in Britain today," Dominic Abrams said.
"Ageism is the form of prejudice experienced most commonly by people in the UK and that seems to be true pretty much across gender, ethnicity, religion, disability - people of all types experience ageism, and indeed people of all ages experience ageism."
The study reveals just how strongly perception of ageing is related to the age of the perceiver, and - to a degree - by the sex of the perceiver, too.
The study may inform better ways to break down classic stereotypes
For example, the arrival of old age recedes into the distance as one gets older.
So, if you are a 24-year-old man, you think old age arrives at 55; but if you are a 62-year-old woman, you consider youth to end at 57.
The fact that criteria "float" makes the task of detecting and tackling ageism particularly challenging, according to Professor Abrams, and has to be considered when developing strategies to undo the usual stereotypes - that old people are "doddery but dear" or that young people are "shallow and callous".
Some other findings from the interviews show:
One key point is that a half of all people under the age of 24 have no friends over 70, and vice versa. And the data shows that those without intergenerational friendships are also more likely to hold negative beliefs about the competence of people over 70.
- From age 55 onwards, people are nearly twice as likely to have experienced age prejudice than any other form of discrimination
- Nearly 30% of people believe there is more prejudice against the old than five years ago, and that this will continue to get worse
- One third of people think that the demographic shift towards an older society will make life worse in terms of standards of living, security, health, jobs and education
- One in three respondents said they viewed the over-70s as incompetent and incapable.
"Inter-group contact and positive relationships across the generations seem to be an important mechanism for combating ageist stereotypes," Professor Abrams said.