What should I do if I don't know when I reached puberty, and I want to make up the prayers I missed due to not performing istinja' before prayer?

If the exact time of puberty cannot be ascertained with certainty, one should act upon what is most likely. Regarding performing istinja (definition)' (washing after defecation or urination) before prayer, if the presence of impurity is not confirmed, then previous prayers are valid. If impurity was present and one prayed with it out of forgetfulness or ignorance, the prayer is valid according to the soundest opinions of the jurists. However, if one was aware of the impurity and intentionally prayed with it, then the prayers are invalid and must be repeated.