When Julian was a year old, his mother married Brett Assange,[39][40][41] an actor with whom she ran a small theatre company and whom Julian Assange regards as his father (choosing Assange as his surname).[35][42]

I wonder how often that occurs? Someone taking their stepfather's surname.

@bradley Back in the day (70's/80's), a lot. I know of half a dozen 2nd marriages, where the young kids were given the new dad's surname, just to prevent questions. Most of the kids changed their surnames back when they turned 18. Only one of them kept it.
@DreamHxx interesting, can't say I know anyone that has done so. Though I guess it is probably more common than I realise.