If you think time goes by faster than ever as you get older, you are probably right.
For we perceive a set period of time as passing much more quickly than when we were younger, a study has revealed.
It is not clear why, but possible explanations range from age-related changes in brain chemistry to a feeling of having seen it all before or even the thought that we have less time left to live.
Whatever the explanation, the discovery by Brazilian scientists helps explain why summer holidays seem to last for ever when we are children but pass in the blink of an eye by the time we are middle-aged.
To measure how the passage of time is perceived, 233 men and women aged between 15 and 89 were asked to close their eyes and mentally count the passing of 120 seconds. All ages perceived the two minutes as passing more quickly than it actually did - but the oldest people were the most inaccurate.
The men and women in the 15 to 29 age group counted down the 120 seconds in 115 seconds, on average.
The 30 to 49 age group took 96 seconds but the over-50s took just 86 seconds. This meant the oldest group perceived time as passing 25 per cent more quickly than the youngest did.
The researchers said: "Our study aimed to estimate the passage of time in different age groups, to test the truth of the saying that time passes faster in older people. Our results indicated that the perception of time passage was accelerated in ageing."
The researchers, from the Sao Jose Faculty of Medicine in Brazil, said that it is possible the phenomenon is due to age-related changes in levels of brain chemicals involved in concentration and memory, both of which are involved in estimating the passage of time.
It is already known that these changes interfere with levels of dopamine, which is key to concentration, affecting perception of time. It is also possible the knowledge and experience we gather as we go through life alter our ability to estimate the passage of time.
So, when we are young and trying something for the first time, we savour every moment.
But as we get older, we have fewer new experiences and so time seems to run away from us.
Writing in the journal Arquivos de Neuro-Psiquiatria, the researchers said: 'Novelty has a strong impact on memory. The time it takes to learn something new is always subjectively prolonged, such as the first sexual relationship, the first job, the first trip without parents or the first experience of living away from home.
"When we are reminded of school holidays, or when we learn to swim or fly a kite, the memory seems endless. Most experiences are new to children and most experiences are repetitive for adults. Adulthood does not hold the constant, never-ending discovery of new things that is inherent in childhood."
However, others argue that as we get older, we simply want to make the most of the time we have left.
This need to cram in as much as possible leads to us rushing through things, including tasks such as counting seconds.
Health psychologist Professor Sir Cary Cooper of Manchester University said: "As we get older, we have limited time left, so we don't want to linger. Older people also have more disposable time and want to get on and use it.
"I'm 75, I still work full-time and I'm impatient. If you asked me to count to 120, I would do it so fast."