Must I offer expiation or make up for not fulfilling a vow of charity that I made for myself during adolescence, especially since I do not remember the number of times I violated the vow?

If you uttered what you mentioned, and it was not a mere thought (حديث نفس), then it is a vow of obstinacy (نذر لجاج), and you have the choice between fulfilling it or offering expiation (كفارة). If you are unsure about the number of times, act upon what you most likely believe to be true, and the more cautious approach is preferred. However, if it was merely a thought (حديث نفس), then nothing is binding upon you. Your young age does not affect the ruling as long as you are mature (بالغاً).