I think the point was that the yeast may be killed by the preservatives, not the alcohol, but to answer your question your problem (biggest problem anyway) is in the sugar. you nee to find the mix correct mix of fermentalbe and non fermentable sugars which will give you enough fermentable sugar to make it alcoholic, and enough non fermentable dugar to make it sweet.
that said, there is probably a much more reliable way to get a sweet alcohol brew
and I think the 21 age estimate may be on the high side...
Primary: First AG- Black Out Stout
Secondary I: Air
60 12oz Brown Glass Bottles: California Pilsner
Thinking about: A really Hoppy Red
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SLEEPY DAD BREWERY