In Italy the age of marriage was much higher. Everyone required parental permission, regardless of age. So doubt the marriage took place in Italy if she was born in the US.
Can you tell us where they were living in the 1920 census? What state, what city? If we can locate them in the census, we might be able to back track the family and see what the possible marriage location could be.
Age was something the Italians didn't keep good records on. Often they are off by a couple of years. So she may have been older at marriage unless you have her actual birth record. Or the date of marriage might have been later.
Again, if we can locate them in the census, it will be a good start into identifying what is going on.