old players doesn't mean will have poor performance too as you can see here Ibrahimovic has maintain his performance by playing with several teams in his own career and from Juventus until PSG he always could be made a lot of goals and currently in Manchester United also he has playing with good performance
Most old player of course with an exception to zlatan, I dont know what this man is made of but this guy gets better the more he is older so he could do a great performance with manchester united. Well he has did a great performance, it is just a matter of time until he set his name as a legend in premier league