The Al-Nassr striker will turn 41 this year, though is expected to carry on after the World Cup.
Discussing former Manchester United and Real Madrid star Ronaldo, Martinez told the Guardian: "I have always believed that it is the body that decides when a player retires, but in reality it is about the head. Cristiano has not yet made that decision, even though he is 40 or 41 years old.
"At this level it is not mainly about talent, but about mentality and the ability to withstand adversity."
He also emphasizes that many people make the mistake of judging Ronaldo based on earlier periods in his career.
"The biggest mistake is not judging him based on how he plays today. After the European Championship, it was said that Portugal didn't win because Cristiano played. When we then win the Nations League, the discussion is instead about what Portugal will do when Ronaldo retires."
Martinez believes that Ronaldo's role in the team has changed over time, but that he is still crucial.
"He is no longer the winger from Manchester United or Real Madrid, but a pure nine in the penalty area. We depend on him to create spaces and score goals. What he has done in the national team the last three years, he has earned, day by day."
At the same time, he highlights Ronaldo's attitude towards the national team, something he got a clear picture of early in his assignment.
"When I met him, I wanted to understand how he thought. Players over 30 can sometimes see the international break as an opportunity to rest. But Ronaldo's attitude is always the same: 'I'm here for the national team, whatever you need'."