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Why managers changing drivers?

The topic name is too short. So the question is...

Why managers changing drivers with all attributes = 20, but experience (exp).

Usually, they drop drivers with exp = about 15 and age about 25. Getting new ones - younger drivers with exp = 1.
Do drivers getting significantly slower at age 25-26? Does loss of the experience of 15-20 costs loss of age?
I think drivers in the age of 25 still should be as fast as 19 years old ones.

It's all about salary, easy. 
I have a driver of 29 years old, 553,5k each race, (who I ain't drop because he is a legend for me) and a driver of 19 getting payed around 343k each race. 200k difference, meaning a difference of around 4 million a season, and that's much. More than you win with 12 exp difference.

Ok, from what age drivers gets slower if they are?

They ain't get slower really, atleast I never expierenced that, highly after 30 they may decrease a little bit.

https://igpmanager.com/app/d=driver&id=5180065