There’s a whole generation of children in China (now in their late 40’s and 50s) who were given tetracycline for everything when they were young - misuse of antibiotics was/is quite common in China.
So one sees many adults of that age with dark/grey teeth.
There are people here in their 80s and 90s who have “lines” across their teeth caused by one of the older mycins given to them when they were kids.