Downside would the chances of it being destroyed go up... Its not as safe as something new with airbags etc.... It is old and although reliable now it may turn into a pita car.... after he puts 50 k on it in the next couple years....
There's more down sides to it than that.....GN's are theft magnets and very easy to steal....and anyone knows one at first glance from 100 feet away. GN's can also very finicky; not just anyone knows how to work on one (unlike a Chevy 350 or a Ford 302) and parts can be hard to come by and expensive. Plus there's also a good chance of blowing it up if getting in the boost and one or more cylinders go lean or there's an incorrect/bad grade of fuel used in the tank...and let's not forget your kid will want to run the snot out of it and maybe even race everyone with it....so add to that speeding tickets, increased insurance, the possibility of wrecking the car and worse, the increased possibility of getting in an accident.
My suggestion is to let junior grow up some before turning him loose with something like a GN. Owning and driving a GN IMO is a big responsibility unlike driving a Civic, which is what junior should be driving. If you still want to give junior a GN, get a V6 Regal or one with a 307 Olds and paint it all black.