Help with Duolingo mistake?

Ciao a tutti! Why is my answer wrong?

I majored in Italian (alongside something else) and studied abroad for a summer but I’m continuing to learn, especially my grammar.

Why is “ne” here? Isn’t it used as a replacement word, such as:

Quanti anni hai? = How old are you/how many years do you have? Ne ho 18. = I’m 18/I have 18 of them (years)