That may be so, but older adults don't generate buzz like college aged fans do. The success of a MMO title is more about building a critical mass of players to keep it sustainable than anything else. You want to play the title that you heard about and that your friends are playing because it is social media. With that in mind, targeting a particularly loud audience that has a high level of influence on their peers is beneficial.
None of that is to say that they might be targeting an older audience with the storyline or whatnot, but they would be loathe to ignore the subset of their base that would be most valuable in attracting and retaining new customers.
I don't want to seem like I'm suggesting that the highschool/college crowd are insignificant to the game's success, I just think that rationalizations around a release date aimed at college/highschool students aren't as significant as some would seem to believe.
When I played FFXI, it was commonly considered to be true that the Japanese playerbase consisted largely of adult professionals, not students. When FFXI went live in North America, the majority of players were adults. Interesting but related sidenote: there was no cultural tension between the JP and NA players at NA launch. It was actually a very friendly environment. It wasn't until shortly after Christmas (younger players getting the game as gifts) and then the PS2 launch (more younger players) that the rift started to emerge between NA and JP.
Even in WoW, my last couple of endgame guilds consisted primarily of working adults. This was to include a healthy mix of college/university/post graduate students, but by no means did they constitute the majority. Oddly enough, the most noteworthy minority were highschool students. Out of a raiding core of maybe 30 players, it was odd to see more than 2 that were highschool age. That is by no means to suggest or insinuate anything other than a population sample, nor is it representative of the playerbase as a whole, but it was interesting to note that as you start to move into the more competitive guilds (ie. top 5000 world and up), the age demographic seems to shift upwards with it.
MMO gamers are a fickle lot. They know (or think they know) what they want and what they like and don't like. In most MMOs, it takes a solid minimum of 10-20 hours of playtime to get a feel for the game. If the first couple of months of service for FFXIV are riddled with bugs, server downtime, and unrefined play, SE will lose out far more than they will gain by rushing to release at a time when the market favors the sale of more retail boxes. There are a lot of people who currently play an MMO and are looking to make a change but are reluctant to do so when a new game launches. They're content to wait a month or two and see what everyone else seems to be saying. If that initial player feedback is that SE has blown it, they're going to lose even more.
With the recent issues around CC payment for FFXI (ie. people who would like to play the game but can't because their payment method is being rejected), SE's previous track record with FFXI (both from a game mechanics and a customer service point of view), and the sheer volume of MMOs on the market now compared to the number that were around when FFXI launched, SE has enough working against them. A game five years in production can't afford to launch with a million subscriptions that dwindle to 500k within a year. Personally, I'd rather see SE put their best foot forward on release day and let players make their decisions based on what SE actually wants to offer rather than what they had time to slap together to meet a deadline based on financial motivations.