The longer form année does not imply a special occasion. It is used to stress the period's duration. Roughly speaking, an is the point in time (J'ai cinquante ans), and année is the period (Il faut compter une année pour voyager à Mars). The same goes for nuit/nuitée, matin/matinée, soir/soirée, jour/journée.
Note: a similar distinction is found in e.g. cuiller (or cuillère) vs cuillerée, where the former simply denotes a spoon, and the latter the volume it can contain (spoonful, as in recipes). Other examples include bouche/bouchée and bol/bolée. It is, generally speaking, a way of expressing an entity's extension in space or time.
J'ai cinquante ans(which, by the way, is fifty years, not five) is a statement about the current situation.J'ai vécu cinquante annéesstresses how long it took you to get there. Age and lifespan - although numerically equivalent - are different things, semantically speaking. Hope I'm not confusing you even more ;-) – Aldo Dec 05 '15 at 10:12