>In other words, we can ask 30 or 40 year olds, did you feel unhappy with your identity as much as today's youth feel about their identities but couldn't wouldn't express it?

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.