I have had a 16 years old boy crew for me on the Tornado. As long as he could use his legs for sheeting main or spi, he would do fine for a day of racing, he was small for a 16 years old at the time. Handling the boat on land was a bit heavier, and we had to experiment a bit before we could right it in light winds.

I think the Mittelmeiers was around 15-16 when they started racing the Tornado. They was a bit light for heavy wind at the time, but very fast in lightish winds.
The sheet loads isn't the issue. It's an economical and political question. As long as ISAF doesn't take more initative and follow up on it's own selection of the SL16, it will be the Hobie16 with spi we will see.

How large is the SL16 class? I see that a new SL16 costs around EUR10.000,-

Got to agree with Steve, they should have chosen a design with the features of the Mosquito, where homebuilding and affordability are strong points. But plywood would probably not cut it, as I seem to remember that one of the criteria for the selected boat was that it should be able to be sailed in a game of "bumper boats".. (what's the materials in the SL16 hulls?)