Has anything changed in the last 10 or 20 years?
Why does this affect as many youths as it does today?
No doubt there was unhappiness with one's identity in the past, but I doubt it was at the level it is today. And, in addition, I doubt it's at the same elevated level in all countries. In other words, we can ask 30 or 40 year olds, did you as a kid feel unhappy with your identity as much as today's youth feel about their identities but couldn't wouldn't express it?
Is it kids leading the change, is it kids following societal trends, is it other, what is it?
Labeling - There's one specific example I'm aware of: Girls interested more in "boy" things than "girl" things were once called a "tomboy", but were still accepted as a girl, just a girl with different interests. Nowadays that same kid would be described as transgender and encouraged to transition.
Yes. We as a society have become dramatically more accepting of gay and trans people.
Gay marriage is a reasonable proxy. Back in 2008, and even a relatively liberal state like California passed a proposition to ban gay marriage. Today popular opinion supports gay marriage. Not just popular opinion in California, or nationwide, but even popular opinion nationwide AMONG REPUBLICANS.
This is why the parents that I know today with children identifying as trans are figuring out how to best support and help their children. The parents of trans people that I knew a few years back wound up becoming homeless as teenagers because of the reactions of their parents. A typical such reaction having been, "You are murdering my son!"
This won't be accurate for a few reasons.
First, there are certainly people in denial about these feelings. A society that is more hostile to trans people will yield a society that has more trans people denying their true identity. That should be obvious. Sometimes that denial is only outward which could result on them lying on a poll. Sometimes that denial will even be to themselves which means they couldn't be truthful on a poll.
If you have spent time around young children you have probably learned that it is difficult to understand a feeling without being able to name that feeling. Similarly it is possible for people to be experiencing gender dysmorphia without realizing that it is specifically gender dysmorphia. I have heard trans people talk about having a general lack of comfort in their body or just the sense that they were somehow wrong that they couldn't explain until they later in life realized they are trans.
Also trans people tragically have a shorter average life span due to increased rates of violence and suicide. Simply measuring the percentage of 40 years isn't going to be comparable to a percentage of 15 year olds because many of those 15 year olds sadly won't live to see 40.
“The proportion of mental health–related emergency department visits among adolescents aged 12–17 years increased 31 percent in 2020 compared with the same period in 2019, the report said. Visits for suspected suicide attempts among girls aged 12-17 then jumped 50 percent from February to March 2021 compared to the same period in 2019. The increase among boys the same age over the same period was just 3.7 percent.“
This makes your theory conveniently unfalsifiable.
https://www.amazon.com/Irreversible-Damage-Transgender-Seduc...
[1] https://4w.pub/social-media-trends-indicate-rise-in-gender-d...