Depending on traits, sims can live longer than the average life span (i.e. There is also an in-game 'No aging' setting - these are found in Options -> Game Options. You can change this setting at any point, and it will take immediate effect.

In TS3, you have the ability to control your sims' aging - or, more specifically, to set the length of their life to either Short, Medium, Normal, Long, or Epic.