The most common gender affirming surgery is breast augmentation -- in cis (non-trans) women.

The regret rate is around 20%.

Gender affirming surgeries in trans people have around a 1% regret rate.

Guess which people need approval from a mental health provider...

@joelle There is no minimum age limit, you just need a doctor ok to perform for girls under 18, but read about some getting it as low as 14 as long as you have parental consent.
Then you have some doctors making an off hand remark then recommend surgery you don't need to fix.