Long story short: there is a "time bubble" the game exists in that means no matter how much we do or how far the story progresses, the year doesn't advance, characters don't age, etc. Think about animated TV shows like The Simpsons, Family Guy, and South Park; despite having been on the air for years and years, absolutely nobody has aged a day. Same concept.

Personally, I clock it in at about 2 years. But that's just me.