Transgender (sometimes shortened to “trans”) is an umbrella term used to describe a wide range of identities whose appearance and characteristics are perceived as gender atypical — including transsexual people, cross-dressers (sometimes referred to as “transvestites”), and people who identify as third gender. Transwomen identify as women but have been classified as males when they were born, transmen identify as men but have been classified as females when they were born, while other trans people might not identify with the gender-binary at all. Some transgender people seek surgery or take hormones to bring their body into alignment with their gender identity; others do not.

Younger children may express their gender very clearly. For example, they may say “I am a she, not a he!”, “I am not your daughter, I am your son.”

There are other ways too, the child may express through: Clothing or hairstyle; Choice of toys, games, and sports; Social relationships, including the gender of friends; Preferred name or nickname

Gender identity is how a person sees themselves. It is their own personal experience of who they are. Mainly people stick to the gender binary i.e. man and woman, but these aren’t the only options.

Gender expression is the way in which a person chooses to express himself, this includes, use of pronoun, dresses, body language, voice, etc.

Sexual orientation is an individual’s sexual preference, purely based on attraction.

It is important to first know the facts about the third gender.

There is nothing medically or psychologically wrong with your child. Gender diversity is not a result of illness or parenting style. It isn’t caused by letting your son play with dolls, or your daughter play with trucks.

If your child is transgender or gender-creative, they can live a happy and healthy life. Get support from other parents of transgenders and gender-creative children, or talk to a mental health professional who specializes in the care of transgender and gender-creative children (if available in your community). Indigenous families can talk to a two-spirit elder or leader.

Preferably at the age of 18 years.

Transitioning is the time period during which a person begins to live according to their gender identity, rather than the gender they were thought to be at birth. While not all transgender people transition, a great many do at some point in their lives.
Reference:
https://transequality.org/issues/resources/frequently-asked-questions-about-transgender-people

No. Gender transition looks different for every person. Possible steps in a gender transition may or may not include changing your clothing, appearance, name, or the pronoun people use to refer to you (like “she,” “he,” or “they”). Some people can change their identification documents, like their driver’s license or passport,to reflect their gender. And some people undergo hormone therapy or other medical procedures to change their physical characteristics and make their body better reflect the gender they know themselves to be. Transitioning can help many transgender people lead healthy, fulfilling lives. No specific set of steps is necessary to “complete” a transition-it’s a matter of what is right for each person.
Reference:
https://www.hrc.org/resources/transgender-and-non-binary-faq

1. Don’t make assumptions about a transgender person’s sexual orientation. Gender identity is different from sexual orientation. Sexual orientation is about who we’re attracted to. Gender identity is about our own personal sense of being a man or a woman, or neither of those binary genders. Transgender people can be gay, lesbian, bisexual,
straight, or any other sexual orientation.
2. If you don’t know which pronoun to use, listen first. Listen to what other people who are familiar with them refer to them as. If you must ask them their pronouns, start with yours first.
3. Don’t ask a transgender person what their “real” name is. Dead names are often a source of anxiety and should be avoided.
4. Respect the terminology the transgender person uses to describe their gender identity.
5. Learn that transgender persons are not new. They have existed across cultures and throughout history.
6. Know your own limits as an ally. Don’t be afraid to admit when you don’t know something. It is better to admit you don’t know something than to make assumptions or say something that may be incorrect or hurtful. Seek out the appropriate resources that will help you learn more. Remember being an ally is a sustained and persistent pattern of action; not an idle or stable noun.
Reference:
https://glaad.org/transgender/transfaq/

No, being transgender persons do not have an illness. This myth comes from society’s tendency to label people who are different from the norm as sick or ill.
Reference:
https://www.transhub.org.au/101/mental-illness