That being old is what brings in all the experiences, yes he can't run and do the tricks he used to do before right now because of his age but his experience gathered all through his career is what's keeping him going. Old players like Ronaldo knows exactly how to and where to position for the ball when his team gets to the opponent box, that's where the experience comes in. Even at his age, there are difficult chances that Ronaldo will convert that these younger strikers can't. Just watch how these old players positions for their goals.
I think Ronaldo is doing a great job at staying as fit as he is. He still works hard and though he may be perceived as being old, I think he's still quite relevant in the team. If not for anything else, for motivational purposes in boosting the morale of the team.
There was an attempt he made at goal with a header from a corner kick. Springing up and above everyone else, I knew he still got it. Although, it went wide, I thought that was a nice attempt and pretty good for an "old" player.
He may not be as hot as he was in his prime, he still knows what needs to be done and strives to do it.