archive of tweet in case of removal:https://archive.is/h4k29
Seems ironic a browser claiming to be against censorship is now engaging in cancel culture if that's not bad enough seems they preemptively blocked reply's from the tweet on twitter as only "People @torproject follows or mentioned can reply." appears on the live page as of this writing.
After looking at the open letter in question it seems they are censoring all opposing views if this thread is to be believed: https://github.com/rms-support-letter/rms-support-letter.github.io/issues/175, not really related just seems like good trivia.
How about for thousands of years. The aspect of what age you stop being a "kid" has drastically changed in the last 200 years as just over a 100 years ago the US allowed boys as low as the age of 14 join the military. We have also seen and still see around the world where girls as young as 12 years old are allowed to be legally married. The Bar and bat mitzvah are multi-millennial long religious traditions that view people reaching the age of 13 being considered an adult for their society at that point and time.