The Portuguese forward spent six seasons at Old Trafford before moving to Real Madrid in 2009.
Ronaldo will come up against his former team on Wednesday when United face Madrid at the Bernanbeu in the first leg of their Champions League last 16 tie.
The 28-year-old believes that he enjoyed his football more at United because he joined a top club at a young age and was allowed to express himself on and off the pitch.
He told The Sun: "Yes Manchester was a very good cycle for me. I won everything there. It was a fantastic part of my life there.
"And maybe it was the time I enjoyed my football the most because I was so young. To be at Manchester United, one of the biggest clubs in the world at just 18. It was a dream. And to play alongside those players you had only seen before on television, maybe that's why I feel I enjoyed it there the most.
"I'm not saying I don't enjoy it here in Madrid, I do. But I'm older now, more mature. I'm not a dreamer any more like I was when I was young. I'm 28 now so I'm not at the beginning any more. I'm middle-aged!"
Ronaldo warmed up for the last 16 first-leg clash by scoring a hat-trick for Madrid yesterday in their 4-1 La Liga win over Sevilla.