The lowest paygrade that is consistently considered a noncommissioned officer in the United States military is E-5. This is because of differences in who is, and is not, considered an NCO in each of the five branches.
In the Navy and the Coast Guard, all personnel in grades E-4 through E-9 are noncommissioned officers, but both services divide this into two tiers. First are the petty officers, in grades E-4 through E-6, and above this are the chief petty officers, or just chiefs, in grades E-7 through E-9. Warrant officers are commissioned, and are in virtually all respects treated like other commissioned officers save that all of them were once NCOs. Some regular officers were, as well, but every single warrant officer was at least a CPO (E-7) before accepting their warrant and commission.
Petty officers have responsibilities that vary by unit, paygrade and technical competence. An E-4 (petty officer third class) might be a leader of a small work detail, but is most often a technician of some sort. By contrast, a first class petty officer (E-6) might be mostly in charge of a division aboard a ship, or in the Coast Guard, might act as the OIC of a boat. Because advancement in the Navy and Coast Guard is dependent on positional vacancies service-wide, not within the unit as in the Marines and Air Force, it is not uncommon for someone to be working significantly below their paygrade.
Chiefs, on the other hand, tend to have much more responsibility. Their most important role is mentoring junior Sailors and Coast Guardsmen, but they are also the people who train most of the junior commissioned officers. In a division aboard a ship, or most shore bases, there will be an officer, typically an Ensign or a Lieutenant junior grade, in charge, but below him is a chief petty officer (or sometimes a senior chief, E-8, or a master chief, E-9), who really makes most of the decisions within the division. He teaches the new, inexperienced division officer the tricks of the trade, and how to be an effective leader. Occasionally the role will be reversed, with a warrant officer or formerly-enlisted junior officer teaching a newly-selected CPO the finer points, but this is not generally the rule.
In some cases, chiefs actually fill officer positions, especially in small commands. Though they are not permitted to act as commanding officers or officers in charge, some small, special purpose units (like SEAL detachments or riverine boats) may have a CPO that runs the unit in all but name. These will generally have a junior officer who carries the symbolic title, and a chief who makes all the serious decisions.
In the Air Force, however, the NCO ranks start at E-5. A Senior Airman is not considered an NCO, a point which may cause some consternation in joint commands, where that same SrA might be serving alongside USN, USMC and USCG E-4s who are considered NCOs. Also, while Air Force E-7 through E-9 personnel generally occupy positions of greater authority and responsibility than their lower-ranked NCO teammates, the USAF does not draw a distinction between standard and staff NCOs the way the USMC, USN and USCG do.