I was aiming for 1-3 years, you're correct. The length of the main story varies from player to player, so disregard the starting date as an indicator of time. I do not have concrete ages for any of the characters, but Emmitt can drink in Open Mode so he's at least 21.
I kept the ages hazy so I wouldn't paint myself into a corner and the player could project whichever they liked. My personal projection is Emmitt starts at 18, ends Story Mode at 20 and comes back in Open Mode at 22. If I've stated Emmitt is 20 in Open Mode, that is a mistake because he should be at least 21. Do you remember where it was said?