Race numbers should only be transferred with the race organisers knowledge and sanction. It causes race organisers significant problems compiling accurate results and allocating prizes.
Passing numbers to people who are much faster, younger or indeed a different sex will always cause problems. This can lead to significant issues with prizes being awarded to the wrong person and or team. More importantly, it can deny the rightful winners their moment of glory. So let’s keep this sensible folk and think of all of the above before you accept or pass on a number!