No, it's not impossible. Just not as common in NJ when you go back this far. My husband's mother was m. at 17 and his grandmother at 16. But I find it unusual as I've researched early Vannemans. In NJ, 18 was about average for a young woman, but it varied by place and time frame and even cultural norms.

I just followed what I think is this Mary back through census records. Wasn't given enough data to be sure, but only her death record gives an estimate of 1808 for her birth. Various census estimate her birth 1802, 1803, 1805 or 1808. That's a 6 year stretch, and so you have to consider the source, and also look at the original docs, try to verify from a couple of sources.