hi, when you go to the menu > settings > clear private data & select "cookies" to be deleted you'll get a second chance :-) otherwise you can also navigate to https://accounts.firefox.com/signin from a separate device in order to create the account.

Therealmisterbee said

I just install Firefox on my android phone. I created an account for sync application but ... when I entered my age, I entered 5 instead of 54 !! So guess what ... my account can not be created because of my age ! Can I modify the age associated with my email address ? And if yes, how ? Thank you !

I manage to sign in on an other device. Thank you.