Exercising in cold weather could make you look decades younger - take it from an age-defying woman who has been billed as the world's 'most youthful' mother.

Liu Yelin, 50, is often mistaken by the others for her 23-year-old son's girlfriend.

The China-born ex-librarian said she was able to stay youthful because she had worked out every winter outdoors for over 30 years.

The super-toned woman said she exercises every day around the year, but it is the sub-zero sessions that have made the difference.

With glowing skin, a ripped torso and not a single wrinkle in sight, Ms Liu really does look half her age.

The fitness fanatic has recently shared pictures and footage showing her doing yoga on a frigid -40C day in one of China's coldest cities.

She said she was invited by a TV programme to practise yoga outdoors in Harbin, China's 'City of Ice', on January 24 when she had the pictures and videos taken.

Ms Liu said she did not find the task challenging.

'I wasn't scared at all. As a matter of fact, I felt great. It was refreshing to do exercises in cold weather,' Ms Liu told MailOnline.

The mother-of-one explained that as long as one keeps their head and feet warm while outside, doing exercise in sub-zero temperatures isn't as daunting as it sounds.

'But if you swim outdoors in winter and you suddenly feel shivering after a few laps, you must get out of water to warm up,' she warned.

Ms Liu, from central China's Xinyang city, shot to fame last year after posting pictures of herself through her Weibo account on Chinese social media.

The energetic woman, who often appears in bikini, said she felt she was still 15.

Ms Liu told MailOnline that her favourite thing to do in winter is to swim in the lake near her house, and she has done that for more than 30 years.

She said: 'It is true that it is painful to take off your clothes and dive into the water, but you would feel great for an entire day after you finish.

'And more importantly, you would stay healthy for a life time.'

Apparently, swimming in bone-chilling lake has helped Ms Liu have smooth and wrinkle-free skin, which makes her look like she is in her 20s.

On Ms Liu's Weibo account, which has over 150,000 followers, she has documented her tough winter workout regime. In one post, she was lifting weights in her hometown Xinyang in the snow after temperature plunged down to lower than -10C.

Last Thursday, Ms Liu dove into a frozen lake in Harbin. She recalled it was so cold that her bikini was frozen when she came ashore.

Despite that, the outgoing woman said: 'My bikini is my armour. With it, I feel confident, healthy and youthful.'