Age eligibility is determined at the start of the year – the fencers age on January 1st, and you remain in that category for the rest of the fencing season.
As the Australian competition year is the same as the calendar year, this means if you are 12 on January 1st, you will be eligible for Under-13 competitions for the whole year, even if you turn 13 on January 2nd.
Yes, people born early in the year have an advantage. It’s called “relative age effect” and you can read more here: Relative Age Effect
Age Categories that are normally used in Victoria are:
U9, U11, U13, U15, U17, U20, 40+.
However in NSW they have aligned with the Asian competitions:
U10, U12, U14, U16, U17, U20, 40+.
Internationally, the FIE runs events for U17 and U20, which is why you see those two appearing in both systems.
In Fencing, “Cadets” refers to the Under-17 age group, and “Juniors” refers to the Under-20 age group. (Here at the club, we usually mean something more like U12 when we say “junior”)
“Veterans” In Australia means ages 40+, but Internationally a Veteran Fencer is 50+. Similarly “Masters” in Australia is 30+, but 40+ internationally.
“Fencing Up” refers to entering an age category above the one you are eligible. For example, a 10 year old who is eligible for U11 competitions, may choose to enter an U13 competition.
They may do this for greater experience and challenge, or because there are not enough fencers to run separate competitions for the two age groups. If they have traveled a long way, they may be looking to get more fencing for their money by entering 2 events.
Generally it is not advisable to fence more than one age group up.
This article discusses the challenges of moving into a new age category: https://academyoffencingmasters.com/blog/what-happens-as-you-age-into-new-age-category-in-fencing/
For Junior fencers the age category can also affect the size of weapon they will compete with.
In Victoria, U11’s must fence with a size 3, while all older categories permit size 5 weapons (full size). Always check the requirements of a specific competition, especially when traveling interstate or overseas.
If you will need a size that is different to what you normally train with, try and practice with the competition size for a few weeks prior to the competition.