If you want hard science for this question, I am afraid we are all lacking. That being said, I've been operating under a yeast centric theory that has seemed to work out so far. There are basically two rules I follow:
1. Unstressed, completely blissful yeast make mead with less fusels.
2. Some yeast naturally produce less fusels or have a broader range of tolerated conditions. This is a genetic/biochemical component specific to each yeast.
Based on the above theory, each yeast has a specific set of conditions required to make it completely happy with absolutely no stress and no fusel production. This would include temperature, ph, nutrients, type of honey, etc. My approach has been to set a specific condition and find a yeast that is very happy with those conditions (Wyeast 1388 in my case).
Now, back to your questions. JAOM uses the above to some degree, but also relies on sweetness and spices to hide fusels. If you have any doubt of this, add 3 lbs of honey or no spice. It is terrible in 3 or 6 months. Even if you can't taste the fusels, they can still give a nasty hangover though! The above is why I test yeast without spice and fermented to dryness. Nowhere for the pesky fusels to hide!
Concerning an 18% no age mead, it can be done with perfect conditions and the right yeast. Admittedly, it will not be reproducable every time and age won't hurt it. I've only pulled this off once (See Ames Farm "Put on your Bigboy Pants") It is delicious now, though the sweetness is probably hiding some fusels. Even if you don't taste the fusels, they can still give a nasty hangover! That statement is worth repeating! As a result, I will age this mead before I drink it.
I hope that gives you the answers you were looking for!
