Other cosmetic surgeries are already allowed, despite the pain and, what can be viewed as, medically-induced deformation and/or mutilation of the body. The pituitary gland controls aging. While the Chinese woman who looked 8 in the news is probably the first thing people think about, the gland can be removed or deactivated (laser?) at a later age, perhaps around 20 or 50. State laws for abortion would probably apply for this surgery for minors - in some states minors around 16 could elect to have procedure, for example.
There is no evidence the pituitary gland controls ageing.
If you've got a pituitary tumour, it can stop growth hormones being released, and without treatment you wouldn't grow. You'd still age normally. You may look like a child even when you are 19 like one Chinese woman, but will still age when the ageing process begins.