More to the point (and already raised by Kegg), young people read comics based on cancelled tv shows? 'Twould be nice to think so ... but I doubt it.
The very young won't be the target audience at any rate, but the people who do remember those shows will be.
Since it'll be on SyFy, the target will be the SyFy audience. The new show needs to appeal to them, since they are the easiest to market to, since they'll already be watching the shows where the ads will run. This show needs to appeal to the Warehouse 13 and Alphas audience and it wouldnt hurt if the Ghost Hunters crowd might check it out, too.
It would be rash of SyFy to assume a large proportion of the current SyFy viewership has even seen B7. (It also would be smart of them to find out this little detail, and to have done so before they signed on the dotted line, assuming it factored into their decision at all.)
And I said nothing about Sy-Fy attempting to make the show for the older fans, I just said what demographic American shows are made for.