Arnold didn’t Realise Joseph was His Until He was Eight.
To begin with, Schwarzenegger had a suspicion that Joseph was not who he seemed to be. The truth is that he was able to put the puzzle together in a short period of time. Because of this, they both insisted that the child’s mother and father were his biological parents. There’s a chance Arnold knew he was the biological father of the boy, but he chose not to acknowledge it for fear of the reaction.
If this did happen, it’s possible that it did. He didn’t have to think long before he realized he couldn’t keep denying the truth. According to reports, Joseph’s resemblance to Arnold Schwarzenegger began at the age of eight. In the wake of realizing just how similar he looked to the bodybuilder, Arnold was forced to accept that he was the child’s biological father.