What must I do and what is the expiation due – if any – if I do not remember the number of years I missed fasting in Ramadan after puberty, and should I estimate them as a full year, given that I did not possess sufficient religious awareness at the time?

If there is doubt about the time of puberty, then investigation is necessary, and one should act upon what is most likely. If no specific time is clear, the default is that puberty has not occurred. If there are two possible times, puberty is attributed to the later of the two. After puberty, the days that were missed must be made up. If the number of days is unknown, then one must make up what ensures certainty of absolution. No expiation (Fidyah) is required for delaying the makeup if the questioner was ignorant of the ruling.