Valee Pancharoen watched her son transform as he became a teenager, first painting his nails, then wearing a wig and, finally, the dresses he had been wearing for years but hiding from his parents.

Now 18, Ponchalearm’s changes are all the more striking as he sits, slim and ladylike in a top of white satin and black lace, next to his stern and athletic twin brother.

Ponchalearm’s aunt absent-mindedly runs her fingers through his waist-length auburn hair extensions as the family quietly discusses whether Ponchalearm is ready for a sex change operation. "It’s my life and I’ve decided that I must do it before university," Ponchalearm says. "I feel happy, it’s fun, I can express myself as I want. I’m lucky I have friends who understand me."

As he speaks, his mother softly cuts in to express her concern that he is too young to understand the long-term consequences of his decision.

"Let’s consider this, you’re still too young," she says. "I want him to take time to grow up a bit."

For two years Ponchalearm has yearned for the operation and at 18 he has reached the age at which, under a new Thai health regulation, he can legally make the decision himself.

The regulation was introduced in April after health authorities became alarmed by stories and the subsequent public debate about underage boys seeking — and receiving — castrations as the first step toward gender reassignment surgery.

Some gay activists and parents worried about potential side effects of the operation on bodies that are still growing believe the age at which youths can independently make the decision to be castrated should be raised to 20.

"They’re trying to do everything to make themselves look like a real woman," said Nathee Teerarojanapong, head of the Gay Political Group of Thailand.

"Why can’t they forget about the external beauty and look inside?"

Thailand is believed to have one of the largest transgendered populations in the world. Academics estimate at least 10,000 live in Thailand, though other guesses are more than 10 times higher. Surgery to remove the testicles apparently carries very little inherent physical risk but the side effects of the hormone drugs that must be taken to hasten the transformation from male to female can include hot flushes, weight gain, muscle loss and a loss of libido, as well as tiredness.

Aside from the psychological impact of that such profound change can bring, some doctors who are expert in gender reassignment insist that any fears of physical side effects are exaggerated.

"There are no serious side effects," said Thep Vechavisit, a leading Thai sex change doctor. "People are stupid and talk about things they know nothing about. These activists are hurting transgendered people."

Current precautions, which require men to consider their decision for one year and see a psychologist for assessment of their suitability and mental readiness for the operation, are sufficient for weeding out the half-hearted, he said.