It's possible that:
* This is a fictional name: There may be a character named "William Rothlein" in a book, movie, or other work of fiction.
* The spelling is incorrect: You may be thinking of a different name with a slightly different spelling.
* The information is private: If this is a real person, their birthdate might not be publicly available.
To find information on a person's birth date, you would need to look for public records like birth certificates or obituaries.