You can email
MTBA about this. They can provide international insurance to cover the race. Cost depends on number of applicants, which normally means it isn't finalised until close to the event.
While not the closest airport, Milan (Malpensa) seems to be the best from Australia. It is then about a 2.5 to 3 hour drive to Finale Ligure. Trains in Italy are also very good. While Nice, and even Genoa, airports are closer, the flight schedules mean it is longer travel time by the time you actually get to Finale Ligure.
You original comment on this was correct. The U23 decision was made to fit in with the normal UCI categories. We also found that it provided a more level playing field for the younger riders, because those 23/24 yo riders were really there in the "Elite" end of the field in a lot of cases, so better to put them with the higher age group.