I think this can be answered with reference to the canonical, best selling young adult fantasy novel series, Harry Potter, loved by 12 year old boys all over the world. In fact my son who is 9, has read the whole series twice, I had to put them away so he would read something else.
There is almost no aspect of the Harry Potter setting related in the books that is not immediately engaging to the young mind, both female and male and is instead superfluous. While some details included may not be entirely essential to the plot, I think this series demonstrates that a lovingly crafted, detailed world that is well aimed at its audience will, all else equal, beat a less detailed equivalent. This is not to say you cannot write a good fantasy story a 12 year old boy will appreciate in low-fi, but it probably won't be as good.
I believe that a key part of Harry Potters appeal to the young reader is the carefully developed relationship between the world of Muggles and Wizards. Every young body reading it is accordingly transported out of that actual muggle world he inhabits and the Wizarding world. The fact that Harry starts his journey in the Muggle world and regularly returns to it serves to anchor what is a highly fantastical setting to more mundane events which the 12 year old reader instinctively understands.
Another key element designed to draw in the young male reader is Harry's sporting activities in the highly dangerous sport of Quidditch.
Finally and most importantly the books carefully develop a series of antagonists which at the beginning of the series are basically schoolyard enemies/bullies and unpopular teachers, immediately recognisable figures of menace to any pre-pubescent child. The way these initially slightly menacing individuals grow into much more real threats throughout the series is very well accomplished.
So with reference to your setting, is there any reason a Fairy cannot use a mobile phone (at least when he or she is present in the mortal plane)? How has interaction with modern man changed the fairy world and economy. I suggest that if you develop a background for the fairy world as it was in the 15th century and then work out how it has co-evolved with the modern world, you can derive an interesting setting.
This is not to say your fairies need to employ technology, but they may have evolved to display certain similarities of culture or economic organisation to that which is familiar in the normal world today (could fairies develop a non technological version of social meida?).
However you do need to find some method of grounding your world on concepts and characters which have traits which are very familiar symbols to your target audience. The schoolyard enemies and friends, the mobile device/tablet and social media, parents and siblings, video games, school/lessons, holidays, figures of authority etc.
These symbols will become more effective where they are integrated carefully into the world building of your setting such that the more potent and recognisable or crucial to the plot the symbol/archetype employed is, the more you invest in the world building behind that aspect.
3Skip or gloss over anything that seems like it could best be answered with the [tag:hard-science] tag. – Frostfyre – 2015-09-03T14:37:59.193
2Fairy tales focus on story, not realism. What needs to be realistic are people's reaction to situations, not the fact that dragons exist. Look at game of thrones. There is plenty to define it as a fairy tale, and yet the story works (though I'd leave out some parts considering your audience ;). – Neil – 2015-09-03T14:50:52.520
1Related, of course. – HDE 226868 – 2015-09-03T16:24:08.143
Give vague references in your story then ask StackExchange - how does the economy in Dark Lord work? – Hannover Fist – 2015-09-03T18:28:24.487
@HannoverFist I dont care about economy in someone else's work. My question basically is: Should I care how economy works, when the story is intended for 12yo? – Pavel Janicek – 2015-09-03T18:30:33.073
@PavelJanicek - I was just using that as an example. I meant that people will fill in the blanks where necessary. Star Trek never explained how their transporters worked - physicists and engineers developed theories afterwards as to how it would work. – Hannover Fist – 2015-09-03T18:43:36.507
How about none? It's your world and it can be as realistic or unrealistic as you want it to be. Everyone is affected by gravity but then all the sudden the trolls make anti gravity potions. Or your world is magic and people needs wands to do trickery (except the elderly of course because they rock) – BlueWizard – 2015-09-03T20:52:59.440
Since you totally enjoyed it, were is the problem ? Your experience just prove that it is possible to enjoy a story, even when being aware of the fact it is unrealistic. So the world do not need to be realistic at all (Alice in Wonderlands or The Little Prince are good example totally unrealistic worlds). – Kolaru – 2015-09-06T01:10:38.740
@Kolaru they are also very old. This does count. – o0'. – 2015-09-06T13:27:59.057