Terry, who has repeatedly fallen foul of the FA, has revived his career under Mourinho, with the defender returning to prominence after losing his place in the side under former Blues boss Rafa Benitez last term.
"He's recovering his self-esteem," Mourinho told The Guardian.
"In the last few years he was not playing a lot, he had problems on the pitch, he had problems outside the pitch, he had suspensions for different reasons, he had injuries, he had managers who didn't trust him enough. And it looked like his career was going in the wrong direction.
"Even I was questioning, from far away, what was happening: physical problems, psychological problems, what is going on? I'm happy he's proving he's still a top player.
"He plays in a position where age doesn't make a huge difference. It's a position where players rely more on positioning, on reading the game and being in the right place at the right time. Experience helps.
"Look at how many top teams have experienced players at centre half: Barcelona and Carles Puyol; Manchester United and Rio Ferdinand; Jamie Carragher at Liverpool was playing until last season and was important. You go to many clubs, top clubs, and central defenders are 30 to 34. John is proving his quality. With what he did at Chelsea in the last decade, I think he deserves to be back on track."