If you are under age and you marry an 18 yr old U S citizen would make it easier to get residency or citizenship?

Assuming that the person underage is illegally in the USA, no it does not. Immigration and marriage are a totally different matter. Okay, sometimes they go together in the case where an immigrant is involved, and is essential for him/her to immigrate legally, but they are tottaly two different things. In order for an immigrant to immigrate to the USA legally through marriage he/she has to go through immigration first (DHS, formerly INS). Many people mistakingly think that once they marry a U.S. citizen they become legal immigrants and/or citizens like a magic act; that's is not true. If naything, it complicates things later on when trying to get their visas and/or permanent residency. Remember: Immigration first, then marriage; not the other way around.