Around 5 years old, horses become stronger with their adult morphology, and when they realize it, you can face some difficult moments 😉
It can last until they are 8 years old, but it really depends on each horse.
Some horses start their 'crisis' at 8 years old whereas some will never do it.
It depends on the lifestyle of the horse, his breaking, his education,...
To deal with it, I don't think you should change all your habits, it's better to have the same routine, to work just like you would do if he wasn't acting crazy sometime.