Officially, less than a year. Here's a pretty in-depth post on the timeline.
Most Eorzeans just use "year." Occasionally you'll see "twelvemoon." NPCs have used a combination of "years," "summers," and "winters" to describe their age. This post has a list of Eorzean units of time and measurement that you might find helpful!
Hope this helps! ^^
edit: oh hey Franz beat me to it! lol
Woops! Well now I don't have to go digging! Thank you Sounsyy!