The sooner that transition is begun, the better the result overall. This is because it takes time for the body to completely masculinize or feminize , and the younger a person is, the closer to androgyny, and the more changeable is the flesh. After age 30, there is little effective difference over time, as the body has become fully mature, and there is no longer any special 'youth benefit' to assist transition. This does not mean that the older transsexual cannot succeed in becoming a socially acceptable and ultimately comfortable member of the sex they were meant to be, but it does mean that the chances of being truly passable, attractive, and content are significantly less. The reason for this is intrinsic to our fetal development.
All human fetuses start out as a quasi-female creature.
The base human state is essentially a stock female form with incompletely differentiated ovaries. Within short order, an influx of sex hormones switches on genes that begin to alter the stock female human form. For the genetic female , the stock form is further refined, as the proto-ovaries complete their formation, and the other sexual organs develop further as they already are. Thus is a girl constructed. For the genetic male baby, male sex hormones begin an astounding and dramatic mutation of the base human form.