Why are teenagers so impressionable or influenced

My 13 year old daughter says she wants to be a boy. That is atypical for them. Is that a phase?

Preface
The subject of teenagers wanting to change the sex is very sensitive and available without much information. There are many people out there who want their ideological point of view, but that doesn't help when you're dealing with someone you love in a real-life situation.

The key term that is being discussed is Gender dysphoria .

Gender dysphoria involves a conflict between a person's physical or assigned gender and the gender they identify with

An article in Quillette that I think your daughter should read states:

Over the past decade, however, a new portrayal of gender dysphoria has suddenly spread, where teenagers or tweens identify themselves as transgender out of the blue without a past childhood uncomfortable with their gender. Experts have identified this presentation as Rapid Onset Gender Dysphoria and are starting to investigate it.

The American Psychiatric Association states

For many children, the feelings do not continue into adolescence and adulthood.

WebMD states

People with gender dysphoria have a higher rate of mental illness

The previous three statements are made for no other reason than to indicate the seriousness, complexity, and gentleness with which this problem should be treated. They also serve as additional resources that a reader can use for their own research.

With this minimal amount of information on the subject, I want to offer some reasonable information on how to deal with it. I'm going to the anecdotal research of a social worker experienced in these matters. The source information is not long read and I suggest that you just read it yourself. There are 2 posts all about having a teenager with gender dysphoria (the author is the same author who wrote the article in Quillette above):
Part 1
Part 2

Summary
Don't fall into the trap and argue about gender dysphoria. Just remember how much you love your child as you go through this. Try to ask questions and understand why your child is feeling this way.

The ideology of innate gender identity often doesn't make a lot of sense, and a lot of young people seem to be sensing this

In any case, avoid embarrassing or punishing your child for their gender identity. Simultaneously can You choose not to consent to the identity. Choosing your answer must be an unequivocal decision based on personal information that no one here will have. This is discussed in more detail in Part 1.

Questioning a child's beliefs should always be done without anger. As much as possible, it should be done with respect and real empathy for the child's suffering. Some parents have had good success with carefully coordinated, brief, reality-based statements made with authority but not with anger

Remember that children are children because they are simply not ready to live alone. They don't know enough and they are not mature enough. It is your responsibility to take responsibility and protect your child during this time.

It's really hard to research this stuff on your own when your world feels like it's falling apart. I firmly believe that the content of Lisa Marchiano in the three main links is appropriate. It's all I could find too. This is a very new phenomenon.

Note on social media
There is strong evidence of the negative impact social media has on teens and tweens. That is not the main issue, but from your question it seems to have a huge impact on this particular situation. There are other questions that have answers on this topic.

More personal to you
I could give you many more opinions, but there are many questions. I wonder how much you know about the therapist you picked, how much you know about what your daughter is doing on social media and, of course, what key issues got the therapist started in the first place. There is a lot to unpack, my questions have to be answered rhetorically.

Appendix 1: American Academy of Pediatrics
Further reading is a PDF printed by the American Academy of Pediatrics' Chicago Children's Hospital. They officially support gender change confirmation in children, but there is a group of pediatricians who actively oppose this position.

If your daughter is considering therapy in the future, with or without your consent, please read pages 39-41 and review the side effects of testosterone therapy.

Irreversible effects of testosterone therapy:
Lower voice Increased hair growth
Mustache and beard growth
Male pattern baldness and baldness
Genital changes
Loss of fertility

Side effects of testosterone therapy;
Cardiovascular risk
Increased weight gain
Elevated triglycerides
High blood pressure
Insulin resistance
Hepatotoxicity (liver disease)
aggression
irritability
a headache
acne
Polycythemia (increased changes in heart attacks, blood clots, and more)
Increased risk of breast cancer
Increased risk of endometrial cancer

Note after research
It seems that there is good support for gender dysphoria in some national groups like the APA. However, no reading of the definition of gender dysphoria has proven to be anything other than endorsing what people feel. I cannot find any real objective information on the subject.

Post Research Note 2
Parenting Stack Exchange is not the place to start questioning gender dysphoria in general. I posted an admittedly lengthy question on the Psychology & Neuroscience Stack Exchange website addressing some of my concerns about the general validity of the APA, which is an attitude that supports and validates gender reassignment, especially among children.

Post Research Note 3
I think there is a huge gap between the reality of this issue and the way we talk about and understand it. I posted a position on this loophole here on the Psychology & Neuroscience Stack Exchange website.

Harvest moon

Many Thanks! I'm really excited to see that first thing this morning! Lots to read, but there is light and it gives me hope. At the moment she was spraying on the ax spray I can smell from her room and I fear that smell. But I have to start with it first. And pray. Oh how hard that is.

Anna SdTC

Only time will tell if it's a phase or not. However, if it's not a phase, you can harm your child by not accepting them for who they are. Trans teenagers whose chosen name is respected are at much lower risk of major depression and suicide. news.utexas.edu/2018/03/30/…

Stu W.

That is an imprecise statement.

dn3s

"Rapid Onset Gender Dysphoria" is pseudoscience.