That is according to pundit and former Chelsea and Scotland midfielder Craig Burley.
Ronaldo has scored just four goals in La Liga this season in 11 appearances.
He has recovered his form slightly in recent weeks, netting five in his last five in all competitions.
But he failed to have a positive influence on the first El Clasico of the season today, as Barcelona ran out 3-0 winners.
Following the game, Burley was asked the reason behind Cristiano Ronaldo’s struggles.
ESPN presenter Kevin Negandhi said: “Real quick, you mentioned he wasn’t sharp, he hasn’t been sharp. What’s wrong with Ronaldo?”
Burley believes the answer is simple, saying age is catching up with the 32-year-old.
“Something that comes to us all Kevin,” Burley responded.
“It’s called age. It slows us all down, it catches up.
“Mother Nature, you can never beat it.”
Real Madrid are now 14 points behind Barcelona in La Liga but do have a game in hand on Ernesto Valverde’s side.