Here is accurate information for you to share with people as needed. Caveat: I am not a medical professional. This is based on my personal research, including talking to other transgender people. I personally have only undergone social transition, as an adult.
There are four basic types of transition: Social, and three medical stages. A transgender person does not have to do any of these to be valid, including social transition.
Social
Minimum age: Any
Permanence: None
Negative side effects: None from the transition
Involves: Using names and pronouns specified by the individual. Allowing children to dress as they please in order to express their identity (within legal parameters). Can be stopped at any time. Even legal name changes can be reversed (by going to court again).
Medical: Puberty Blockers
Minimum age: 8 (with rare exceptions)
Permanence: Virtually none
Negative side effects: Low
Involves: Taking drugs that postpone the onset of puberty. This suppresses the further development of adult sex characteristics, such as (in males) facial hair and deepened voices and (in females) menstruation and breast development. Cessation of these causes puberty to proceed as normal, albeit delayed.
Medical: HRT (Hormone Replacement Therapy)
Minimum age: 14 (with very rare exceptions); 18 strongly encouraged
Permanence and negative side effects: Moderate
Involves: Taking hormones (males who are women/femme: estrogen and progesterone; females who are men/masc: testosterone) to cause the development of adult sex characteristics associated with the person’s gender. Cessation of these has varying impact, depending on the person and how long they were on HRT.
Medical: GCS (Gender Confirmation Surgery)
Minimum age: 18 (with very rare exceptions)
Permanence: High
Risk: Moderate compared to other surgeries
Involves: Receiving surgery to modify the body, including genital removal and reconstruction, breast modification or removal, and facial/throat modifications. Surgery always come with risk and is permanent. The regret level of GCS is low compared to other surgeries. However, because of the risk and the permanence, it is generally only recommended after a significant period of psychological therapy, and is only allowed for minors when its absence creates a significant risk of suicide, extreme self-harm, or other medical emergency in the opinion of medical professionals.
Additional comments
Note 1: Side effects list the effects of the transition alone. In all cases, the risk of bullying and stigmatization from transitioning should be weighed against the internal distress from not transitioning.
Note 2: Puberty blockers and HRT were originally designed for cisgender people. Estrogen-based HRT, for instance, is in common usage for menopausal cisgender women whose bodies have decreased natural estrogen production. Puberty blockers were developed decades ago for “precocious puberty”. Ironically, while puberty blockers for transgender children are rare before age 8 or so, they are recommended for cisgender children younger than that when they are experiencing early puberty, and yet there has been no social outcry about that.
Note 3: This overview uses “male” and “female” as indicators of sex, not gender. Sex is generally defined by chromosomes and genitals at birth; gender is about self-belief. A cisgender person is someone whose sex and gender are the same; a transgender person is someone whose sex and gender are different.