The current age of Lewandowski isn't affecting his performance at all, because he is still a goal machine scoring many goals as usual. He is the highest goal scorer currently with a total of 19 goals. Mbappe is three goals behind him and if Lewandowski is not a prey to injury before the season runs out, he might be the highest goal scorer. Lewandowski was not on his peak form last season.
Lewandowski is 36 years old, there is high likelihood that he might drop form due to age redundancy in few years time which is natural, but for now he has been firing from all cylinders which is very rare for a striker of his age, it shows that his brilliant, imagine him at that age topping the goal chat when there are younger players like Mbappé who are very vibrant. Obviously if things continue like this, lewandowski will definitely be the height goal scorer in La Liga. Since this new Barcelona era, he is one those players that has helped kept the club afloat.
Lewandowski is younger than Ronaldo and he is older than Kane but they are players with an age that is no longer productive but for skill and greatness as striker as well as finishing still shows very dangerous sharpness, Lewandowski could be one of the best player transfers to Barcelona who is able to provide consistent performance for the past 3 season.
Maintaining Lewandowski will be hope for Barcelona considering that in Barcelona less than ideal conditions Lewandowski can still play very well, I think even some young players still can't surpass him, however experience and mentality will always be the main factors in maintain greatness.