I believe this to be true as well.
Furthermore, can I say this: Man Utd. have had successful 40+(ish) aged players in the past e.g. Teddy Sherigham and Ryan Giggs. Teddy was never that quick even in his 20s! whilst Giggs was very successful at adapting his game. He wasn't fazed by playing less minutes and in a different, less intense role, to extend his career beyond that of most other players. I doubt very much Ronaldo will do that - we know from last season he wanted to basically be the 1st name on the teamsheet and there were even rumours it was contracted that if fit he plays, although I myself doubted that. But even though it wasn't in black and white, it technically became so once Sir Alex in effect criticised OGS (after the Everton game I think) when Ronaldo was on the bench at kick-off.
Finally, I recently read statistical analysis to show (I can't find the link now) showing Man Utd scored something like 20 goals more in the season before Ronaldo joined so even though he personally had a successful season, his impact on the team - on the pitch and and behind the scenes in the dressing room had, overall, a detrimental effect on Manchester United.
Having said that, because of other issues to do with loss of form and unavailability of up and coming strikers at United, I do think he needs to stay until Christmas at least, possibly followed by a move to America, which would be the best for both parties.