Emile Ratelband, aged 69, wanted to change his birth date by 20 years to avoid what he called discrimination.
"We live in a time when you can change your name and change your gender. Why can't I decide my own age?" he said.
But the court disagreed, highlighting that many rights in law are based on a person's age, and changing it at will could cause many problems. There was no legal basis to make such a change, it said.
"Mr Ratelband is at liberty to feel 20 years younger than his real age and to act accordingly," the judges said, but changing his legal documents would have "undesirable legal and societal implications".
Mr Ratelband, who calls himself a "positivity guru", made headlines around the world with his unusual request. Ahead of the hearing, he made TV and other press appearances, saying he felt discriminated against in both employment and on the popular dating app Tinder - and said his doctors had told him he had the body of someone in their 40s.
"If I'm 49, then I can buy a new house, drive a different car. I can live again” he said. "When I'm on Tinder and it says I'm 69, I don't get an answer. When I'm 49, with the face I have, I will be in a luxurious position."
The court stated "Rights and obligations are also attached to age... for example, the right to vote, the right to marry, the opportunity to drink alcohol and to drive a car," the court said.
It found that the possibility of declaring oneself younger could open the door to the opposite - becoming older and cause "all kinds of legal problems".
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Adapted from BBC news https://www.bbc.co.uk/news/world-europe-46425774
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