Also depends on what needs to be assembled. For example, if the bike only requires installing the pedals, turning the handlebars and tightening it, etc., then you should be able to do it yourself. However, you'll have to make adjustments so the bike fits you properly eg. seat height, bars.
Anyone who can manage a can opener should be able to final assemble a fixed/single speed.....even your know-it-at neighbour shouldn't complain too much helping out......