Pertinent description of autogynephilia & how & when it starts by sexologists J. Michael Bailey PhD & Ray Blanchard PhD:

Autogynephilia is a male’s sexual arousal by the fantasy of being a woman. That is, autogynephilic males are turned on by thinking about themselves as women, or behaving like women.

The typical heterosexual adolescent boy has sexual fantasies about attractive girls or women. The autogynephilic adolescent boy may also have such fantasies, but in addition he fantasizes that he is an attractive, sexy woman.

The most common behavior associated with autogynephilia during adolescence is fetishistic cross dressing. In this behavior, the adolescent male wears female clothing (typically, lingerie) in private, looks at himself in the mirror, and masturbates.

Some autogynephilic males are not only sexually aroused by cross dressing, but also by the idea of having female body parts. These body-related fantasies are especially likely to be associated with gender dysphoria.

Males with autogynephilia can have a variety of autogynephilic fantasies and interests, from cross dressing to fantasizing about having female bodies to enjoying (for erotic reasons) stereotypical female activities such as knitting to fantasizing about being pregnant or menstruating.

One study found that autogynephilic males who fantasize about having female genitalia also tended to be those with the greatest gender dysphoria.

https://4thwavenow.com/2017/12/07/gender-dysphoria-is-not-one-thing/