The other potential explanation I can think of is the footnotes were added after the fact. Like Jing Yuan just has this giant stack of preceptor letters shoved somewhere and Qingzu is going through them trying to figure out what needs filing vs what gets burned. But it's a bit of a stretch...

I think there's enough normal Vidyadhara to balance out that one girl. Much like the human average life expectancy isn't going to suddenly change due to one case of early infant mortality. Qingzu's age just seems like an oversight on the writer's side.