Keeping the game accessible to even new players is a priority for Square, so its not a big deal if you wait. But if you start now, you can work on getting some non-optional things out of the way in advance, and there's plenty of things to enjoy right now if you do have some time.

Note that you'll need to be level 30 to unlock the new jobs, and you'll have to have completed all the currently existing main story (several days of questing at least, once you're max level) to get to expac areas.

tl;dr: There will never be a "bad time" or "too late" to start.