The court's holding that the punishment was disproportionate was based on state law grounds, not the federal constitution. The 8th Amendment's ban on cruel and unusual punishment is not involved (as the trial court noted).
The trial judge also ruled that the law prevented a 20-year old from getting a FOID and it would therefore violate due process to charge him with having a firearm without an FOID. An alternate interpretation is that this is exactly what the Illinois legislature intended -- to make it unlawful for anyone under 21 years of age to possess a handgun. I would think the state might consider appealing the decision if this were the only issue. They may not since the case has no precedential value (at least as far as I know).