The noise that shows if someone is above 25

Can you hear it?

(NI QIN/iStock)

How can you guess a person's age?

A glance at the way they dress, the colour of their hair or the lines on their skin might give a clue - but the answer partly lies in their ears.

Or, using a special sound, you can at least pinpoint whether someone is past their mid-twenties.

This is because those over the age of 25 on average cannot hear what is called a mosquito tone – or a noise above the 17.4kHz. Click here to listen to it.

The phenomenon is caused by age-induced hearing loss called presbycusis, which occurs when the hair cells in the cochlear of the ear wear down the more they are exposed to sound.

For most people, this hapens in by the time they are in their mid-twenties, after which point they can no longer hear the tone.

The noise is omitted from a device known as The Mosquito or the Mosquito alarm, which is used to stop young people from gathering in public places.

However, campaigners have called for them to be banned and labelled them as unethical.

Liberty, 11 Million and the National Youth Agency have teamed together to form Buzz Off, an organisation against the devices, calling them “aggressive and unpleasant” on their website.

“The Mosquito creates a no-go area for children and young adults who may have done nothing wrong," the organisers said.