OK, what is the definition of 'Best friend' in the context of this discussion ? Do you mean being very close and still avoiding sex or sexual behavior between two parties ? or is sex also included in this 'Best Friend' domain ? Please make it clear first. .. .

If you say being 'close' forever and never into sexual activities ?? Then my answer is .. never never and never.

 A boy and a girl can not remain in a state of 'just a friend' (I prefer it to call  'third state') for a long time. It is unstable state, rather say - a transition state, and soon it will acquire either a state of 'one' or of 'zero'. This is because nature has defined only these two stable states of serious relationship between two opposite sex. You can not go against nature's law which says - nothing in this cosmos can remain in unstable state forever.

 ;-)

And if you try to remain in that 'third unstable state' soon either one or both of you will get into serious emotional and behavioural problem. Any close relationship and trust culminates in the form of sex at the end.. and it is the truth.