Younger organisms are _______ and older organisms are ________

Some organisms undergo evolution, which creates a lot of differences between present organisms and their ancestors. These organisms are known as young as they are not similar to their ancestors.

On the basis of evolution we classify organisms as primitive and complex organisms.

Organisms that look similar to their ancestral body design are called primitive organisms or older organisms.

Younger organisms are complex and older organisms are simpler organisms.

Hence, complex and simpler are  the correct answers.