What do the rules mean by age-appropriate statistic? Does this mean we can't have more complex stuff like ANOVA?

I think it means that you shouldn't be doing mode or range for Division C.

Who knows? You might want Pearson Correlation.

Also, how would ANOVA even apply to XPD data?

Many proctors will be unfamiliar with ANOVA, so I would not recommend using it for your statistical analysis. It is certainly not necessary to do so.

At the Division C level, I would generally recommend you choose either Pearson's correlation coefficient (r and r^2) or Student's t-test. These will cover almost every scenario and are easy to calculate using a graphing calculator. It is important that you understand and interpret the meaning of the results, rather than simply reporting them.

Mean and standard deviation will almost always be the best measures of center and spread to report. Occasionally, median might be preferred, but you're unlikely to have a sufficient sample size to know if your data is skewed.