Ok, first of all, don't worry, this doesn't have any spoilers aside from one which I've tagged(unless I've inadvertently figured out something that happens in the last 3rd of the game which i haven't played yet). Secondly, this really is useless nitpicking/rambling, so if you're not interested then don't bother with it.
So, like many games, you get the occasional chance to rummage through bookshelves and get a better understanding of the game's world. In the library at Daath, there are books about the calender, what they call the months, days of the week, etc. One thing caught my attention. One of the books says that a year is 765 days, each day being your standard 24 hours. Now this may very well be a typo, or it may just be the writers trying to make the world a little more interesting. Unfortunatley, they don't specify the number of weeks in a year or weeks/days in a month to check the 765 number. While it's probably a typo, lets consider it as though 765 was the correct number they intended to list. Looking through the game manual, the character's ages are as follows:
Luke 17
Tear 16
Jade 35
Anise 13
Guy 21
Natalia 18
These numbers are also confirmed in the game here and there, so they aren't adjusted to meet our 365 day per year idea. Now I suppose this could be explained that the people of this game's world just age/mature considerably slower then what we would consider normal. But just for shits and giggles, I thought I would convert their ages to years as we know them(365 day, rounded as necessary).
Luke 36
Tear 34
Jade 74
Anise 27
Guy 44
Natalia 38
If you want to consider the Luke being 7 thing, then his converted age is about 15, which I suppose fits his stupid angsty tendencies better then if he really were 7)