Detransitioners have existed as long as there have been people transitioning. I knew two, slightly older than me, when I was transitioning.
They're not only young though, they can be any age and at any stage during or post transition (Philosophically, I don't know if someone can be through and done transition if they detransition? Maybe?)
Detransitioners were rare because transitioning was rare. There were a lot of barriers in place to realistically prepare someone to live a visibly trans life in a homophobic world.
Before the internet, detransitioners were a tale of caution. Trans people early in transition paid a lot of attention to them because they were messengers from the cruel future. Of being disowned and mocked by family. Of job loss and unemployability. Of physical violence. There was respect of choices and mutual acknowledgement of difficult decisions made.
The major difference now vs then is some detransitioners have allowed their experience to be weaponised against trans people. Trans people have a right to maintain strong feelings about all people participating in the harm of trans people, including detransitioners.