When To Change Puppy Food To Adult Food?

My Chihuahua is 4mnths old{hell b 5mnths on the 17th}, hes currently on puppy food
My moms Cockapoo is 2 years old and is on adult food {obviously} but she, the cockapoo, is on beneful and my dog is on Purina cuz thats what the breeder used. He barely eats it and always tries to sneak and get her food and will sit in my bed with his stomach growling to high heaven but will NOT eat his food {i leave it out all day incase he gets the urge}. It is soo rare he does. My uncle breeds and trains Jack Russles and said not to switch him to adult food till 6 months but i want to know if its true for all breeds or if maybe it varies breed to breed. Any constructive advice is deeply appreciated.