Agree. It is not about his age. Ronaldo seems to have no problem with his fitness. But he may have a problem with his motivation and his adaptation to Ten Hag's tactics.
Ronaldo wanted to leave the club and spent most of the summer on vacation, with limited pre-season training. The lack of training affects his fitness and his adaptation to Ten Hag's tactics.
We all know that Ronaldo has no good relationship with Ten Hag, it is very visible since he rarely plays as a starter lately. He spends more time on the bench and didn't get a lot of time to play as before. He is also not a special player on Ten Hag tactic, he is no longer a player who gets the most assists from others. He must work harder to score now.
Ronaldo is an ambitious player and he will not want to stay on bench in every match. However, in order to get back into Ten Hag squad, he will have to take any chance and make impression. So far, Ronaldo did not achieve any impression but the season is long and I believe that Ronaldo will get his best fitness back and will be able gradually get his position back.