Is it obligatory to make up for missed fasts by the same number of days, or by fasting sixty days for each day missed, given the uncertainty of the onset of puberty, and are there days when it is not permissible to make up for missed fasts?

Fasting is not obligatory until one of the signs of puberty is confirmed. If you are uncertain about having reached puberty, you are not obligated to fast, because the default assumption is non-puberty until it is definitively proven.

If you reached puberty at the age of twelve, you must make up the days you missed fasting in previous years. Expiation (fasting sixty days for each missed day) is only required for intentionally violating the sanctity of fasting by sexual intercourse. As for breaking the fast by means other than intercourse, or due to ignorance or forgetfulness, there is no expiation for it, but only making up the missed fasts.

It is permissible to make up for Ramadan on any day of the year, with the exception of days when fasting is forbidden, such as Eid days, the Days of Tashreeq, and Ramadan itself. The missed days must be made up by number, and it is permissible to make them up at any time when fasting is allowed. It is not permissible to make up for the previous Ramadan during the current Ramadan, as the latter is designated for performance.