Lewandowski, Messi or Ronaldo's fitness is still excellent. And I wouldn't be surprised if they play until 38/39. I agree with you that Lewandowski can easily play until 38. But I don't think he will be effective at 38 for Serie A or Premier League. If Lewandowski can maintain his fitness, he can stay with Barcelona for a long time. Barcelona are going back to their old tiki taka tactics. And so I think if Lewandowski's fitness is good, he can stay with Barcelona until he is 38 and continue his good performances.
Capello said that in Qatar we saw the final death of tiki taka. I am inclined to trust him, because I came to the conclusion that even in it golden years, this strategy was nonsense and gave results only due to the highest skill of the performers. I don't think that Xavi will last long in Barcelona and that tiki taka will be relevant there (although it would be good since I'm a Real Madrid supporter). But in general, this will not prevent Lewandowski from playing until the age of 38-39 - La Liga is not as intense as the Premier League.
Tiki taka was an impressive tactic in its time, but for now many coaches and teams can anticipate their moves and make their defenses difficult to penetrate. This tactic might bring the team to control the match, but just mastering the match is not enough if you cannot penetrate the opponent's defense and score goals. I don't mean to be dismissive and consider this tiki taka useless, it's just that innovation has to be developed with the dominant ball possession they get.