The government ban is effective immediately, with doctors no longer allowed to perform operations such as nose jobs and breast enhancement on minors.
Liposuction and so-called "double-eyelid" surgery, which makes the eyes look bigger, are also prohibited, the Ministry of Health and Welfare said.
Doctors face a fine of up to TwD 500,000 (USD 16,600) and a year's suspension if found in violation of the new rule, the ministry said. They could even lose their licences completely.
However, health officials say that some lied about their ages to get treatment.
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"As the acceptance for cosmetic surgery rises in Taiwan, we are concerned that more youngsters and even parents will overlook the risks and consequences," an official said.
"The ban is necessary as invasive procedures are not good for youngsters who are still growing."
Cosmetic surgery on minors is not covered by Taiwan's national health insurance programme so there is no government data on how many procedures have been performed.
The government acted after calls for a ban increased following reports of children as young as nine undergoing surgeries, some unsuccessfully.