I don't know how deep FD's starforge models this kind of stuff but I suggest a 'rogue planet' theory This gas planet was thrown out of some other system and got caught in stable orbit around this planet.
Now, I've never seen objects in any given system to have different listed age, but it doesn't mean that it's not possible.
400 billion star systems, trillions of different combinations... anything is possible alright. I'm waiting to see a star orbiting a black hole, orbiting a gassy, orbiting a rocky planet, orbiting a giant toaster. That'd be cool.
I don´t know enough about solar system formation, but having such a small gas planet must require some weird stuff taking place.
Another example of this phenomenon was discovered by Iridium Nova on Apr 29th 3302, in the Flyua Dryoae DG-M d8-3 system. With a radius of only 8787 km it was the smallest (but not the lightest) gas giant discovered at the time (and possibly so far).