i dont think the artist ever answers it. Going by the canon, digimon do not age like humans, veemons like shoru only evolve to their adult form (ex-veemon) if they get strong enough to achieve digievolution, otherwise they could stay in thier child like form for decades, even centauries, since in some continuities digimon live thousands of years (they live forever in the manga).
i dont think the artist ever answers it. Going by the canon, digimon do not age like humans, veemons