Can braces straighten teen just as easily when you are over 18 years?

Yes, but... I assume you are asking about dental braces. Orthodontic treatment works best when we can harness growth, therefore at the tween-teen interface. The older you get, the more complex the problem becomes, the less growth there is to work with, the longer treatment will take, and there will be a slight negative gradation of the prognosis and a longer retention component after active rx.
Braces. Its never too late but it can be easier the younger you are in age. It'll be worth it.
Not "as easy..." But braces work at any age. The younger the person, the softer the bone.