The age players are considered old varies and it boils down to many factors such as position and exertion. In football, some positions require more exertion than the other and this plays a lot of factors in considering when a player is regarded as being old. However, if a player is in a less exertion position this may increase his playing career
On the average, a player is usually considered old when he's between age 32-35. Similarly, a player is considered young when he's still between 18-25 years.
Goalkeepers have the longest lifespan when it comes to football career and that's why most of them still perform well in their mid-late thirties. We have goalkeepers like Buffon who still play well in his late thirties. Another is van der Sar who played well even in his mid to late thirties
As for defenders, they play a position with more exertion, therefore, they are considered old in their early thirties. Although there are still many defenders who are still doing well in their early-mid thirties but this does not change the fact that they're considered old when they turn 32/33.
As for midfielders, they don’t normally have as much running to do so you can still see them still playing well in their mid thirties. Midfielders like Xavi and Pirlo were still hot cake even in their mid thirties
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As for strikers they have the highest exertion and that's why they have the shortest football career lifespan. The reason why top clubs barely go for strikers that are in their thirties but the same can't really be a said about China leagues. They still buy old strikers just to develop their leagues. Anything above age 30 is considered old as a striker, although we still have a player like Ronaldo who is still playing well at age 33 because he is an exceptional player
Honestly, these are just my own opinions and I may be right or wrong.
I drop my pen at this juncture and I will like to hear your own opinions on the comment section
Thank you for reading and have a nice day 😎