By age 7

The following is an excerpt from Cedars-Sinai of June 2020 which explains that, on average, gender dysphoria tends to manifest by age 7 which was certainly the case for me. Diagnosed only at 45:

"Gender dysphoria manifests early in childhood and can persist for years before patients undergo counseling and treatment, a Cedars-Sinai study has found. The findings also reveal that untreated gender dysphoria can result in poor quality of life for transgender people, beginning in childhood and lasting throughout adolescence and adulthood.

Gender dysphoria refers to the strong discomfort or distress often caused by a discrepancy between a person's gender identity and their biological sex assigned at birth, according to the Diagnostic and Statistical Manual of Mental Disorders – Fifth Edition. Gender dysphoria worsens as those experiencing it grapple with expected social gender roles and sex characteristics that do not align with, or reflect, their internal sense of gender identity.

Federal and state population studies from 2016 estimate that 1.4 million to 1.65 million U.S. adults – or 0.6%-0.7% of the U.S. population – identify as transgender, according to the Williams Institute at UCLA School of Law.

The Cedars-Sinai study, published recently in JAMA Network Open, a journal of the American Medical Association, was led by urologist Maurice Garcia, MD, director of the Cedars-Sinai Transgender Surgery and Health Program. The study included 155 transgender women – those who identify as women but whose sex assigned at birth was male – and 55 transgender men – those who identify as men but whose sex assigned at birth was female. All of the participants were adult patients seeking gender-affirming surgery.

The study findings revealed that 73% of the transgender women and 78% of the transgender men first experienced gender dysphoria by age 7"