36 years old and still the highest goal scorer in La Liga, his age could be telling but his presence in that squad is of great importance, he has contributed to the team, it's not about agility but at times experience helps the team. Barcelona should find a replacement but not let him go at the end of the season, they should extend his stay by another season then once in a while play him and then another play the new striker, he wouldn't be getting much time on the pitch though but having an experienced striker can help them at a point in need.
That's a much better scenario and not letting Lewandowski leave so quickly without a suitable replacement. I also believe that the contribution and productivity of goals achieved by Lewandowski cannot be separated from the quality of the creative midfielders that Barcelona have so that the combination provides individual benefits to the forwards. Lewandowski's experience is also quite helpful because he understands the right conditions when receiving a pass and then scoring a goal. He is a fairly qualified striker even though he is no longer young and Barcelona itself must maximize the skills he has.