It is ok to say:
I don't eat fast food (a present habit)
I ate fast food when I was a child (a past habit - it no longer happens now)
Now, when we combine these 2 sentences together.
Does this sentence sound strange?
I ate fast food when I was a child but I don't eat fast food
Can we say:
I ate fast food when I was a child but NOW I don't eat fast food?
Can we use "NOW" for a present habit because we mentioned a past habit previously?
USE 4 Habits in the Past
The Simple Past can also be used to describe a habit which stopped in the past. It can have the same meaning as "used to." To make it clear that we are talking about a habit, we often add expressions such as: always, often, usually, never, when I was a child, when I was younger, etc.
I studied French when I was a child.
He played the violin. He didn't play the piano.
Did you play a musical instrument when you were a kid?
She worked at the movie theater after school.
They never went to school, they always skipped class.