.jpg)
Fencing blades come in four sizes: 0, 2, 4, and 5. 0 is the shortest and 5 is the longest. 5 is the standard/default size. I don't know what happened to sizes 1 and 3. I recall seeing an antique foil, long ago, that had a "1" stamped at the base of the blade to indicate size.
Fencers ages 7 to 10 who compete in Youth-10 events must use blades sized 0 or 2 in said events. Any of those fencers who are also eligible for Youth-12 (ages 9 and 10) are not obligated to use blades 0 or 2 in those events. Once a fencer fences in a Youth-12 (or older) event, he or she is no longer obligated to use a size 0 or 2 blade.
In epee reach is key, so I do not recommend size 0 blades for younger epee fencers. Further, once an epee fencer is not obligated to fence with a smaller blade, I recommend fencing with a full size (size 5) blade. Saber and foil fencers may want to keep with their shorter blades depending on their size and arm strength. It is best to consult with one's coach about this. Opinions will vary on the merits of shorter blades. My general guideline for foil and saber fencers age 13+ is that if they are under about 5'2"/5'3", I recommend they use a size 4 blade. The right-of-way in foil and saber will make up for any loss of reach, and the shorter blade will be easier for the shorter fencer to manipulate.
A great read for those interested in varying blade sizes in epee fencing is soon-to-be Fencer Master Paul Sise's "Height Differences in Epee Fencing and Strategies for the Shorter Fencer" to which I have linked below.