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.
Age is before football, that doesn't matter anymore here as long as you are active and you are healthy, the team will keep you but they wouldn't offer a very long contract to you because you will become a liability to them if you become injured or maybe something came up, they wouldn't be able to loan you to another club and they wouldn't be able to sell you as well, that's why many clubs gives a year and may renew if the players has more to deliver.
Lewandowski performance looks like a young guy performance because right now, nobody in Laliga has goals like him this season. Not even Mbape that was hyped, who wouldn't want to keep him. Barcelona is smart for keeping him while training more boys to become a man. Yamal will do better but for now, he need someone like Lewandowski and soon, someone will come and be his boy because I'm seeing that boy as one of the next best project after Messi.