The difficulty for Lewandowski might be that it is becoming harder for him to compete with younger players, but so far he is been doing quite well. He was in excellent form before he got injured, so now he needs to be more careful, though it is easier said than done because anything can happen unexpectedly during a game. Ronaldo by his example shows that age is not always an obstacle to playing at a high level, and Lewandowski is still part of a top club. At the very least he will continue to play for another year at Barcelona, and after that he might receive an offer from Saudi Arabia if Barcelona decides not to extend his contract.
With Lewandowski current age, I believe he is still at his best at Barcelona, he has surpassed other players who are still in their early 20s but cannot score ten league goals in a single season. Lewandowski is 36 years old and will be 37 at the start of the next season. He scored 27 league goals for Barcelona and 41 goals in all competitions last season, which is still impressive seeing as he continues to compete with Mbappe who is still young for the league Golden Boot.
On the other hand, Lewandowski and Cristiano work on the field to turn their chances into goals, and as we can see, Lewandowski is still capable of scoring goals. Ronaldo, on the other hand, cannot be the same Ronaldo we knew before, at this age, he can only score when he sees an opportunity, and he cannot be the same Ronaldo who can break through opponents defenders to score goals. So Lewandowski should go to Saudi Arabia and enjoy the rest of his career.