Depends on the story. A friend of mine entered the working class at 14, had his own apartment and all. His story would fit my definition. My uncle had a very similar story--being kicked out by my grandmother--at the age of 30, and his story fits my definition. But I wouldn't market their stories as NA as the don't give off that same College/Entering the world vibe that a very late teens to early 20's story gives off. There's a thin line with my definition.
The age would matter more with relation to the story, and the reader. Just like the YA is geared towards YAs, NA is also geared towards NAs. Age matters for connection, the constructions of society, and basic story premise--though these can be overwritten.
However, again, NA wouldn't be a field I would market into. Almost all of the NA books I've come across are heavy romance involving 20-somethings. If your story is this, then all for it. But, YA and adult have broader markets, and with some tweaks a story can become either/or.
Research some well-respected NA stories, that are regarded as "pure" NA by readers, and read them to figure out if your story is like them.