Alexander Koryagin:

AK> Paulo Coelho:
PC>> Everyone, when they are young, knows what their
PC>> Personal Legend is.
AK> Should it be  "Everyone, when he is young, knows"
AK> [...]
AK> Although, I found in Wikipedia information that "they"
AK> can be used as single pronoun:

Right you are, but I accept only two cases of such usage:

  1.  General -- when the pronoun refers to persons of
      either sex, as in Coelho's sentence, and

  2.  Circumlocutive -- when the speaker does not know the
      sex of the person in question or wants to conceal it.

Remember also Cab Calloway's "Everybody eats when they come
to my house."

AK> In the early 21st century, use of singular _they_ with
AK> known individuals has been promoted for those who do not
AK> identify as male or female:
AK> "This is my friend, _Jay_. I met _them_ at work."

They had better have saved it for schizophrenics with
multiple-personality disorder.

With the preceding comma, Jay is the person addressed.  Omit
it and it will be the name of the friend.

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 * Origin: nntps://fidonews.mine.nu - Lake Ylo - Finland (2:221/6.0)