27. A young girl whose time to see has not yet come can establish a cycle like any other women with 3 cycles or 4 cycles for an interval based cycle. She is considered to be a minor if she has not attained majority (i.e. 12 years and a day), even if she has signs of maturity, or if she has attained majority but does not yet have signs of maturity.  There is a however, a difference between her and an adult woman.  That is, even if she establishes a fixed cycle, if there is an interruption of 3 onos beinonios, or 90 days during which she does not have a period, she does not concern herself  at all with the original cycle and she returns to her original state of minority.
    
    In the future if her periods resume on the same cycle, she does concern herself with them until they are established 3 times because she is not yet mature and we consider the earlier flows to have been incidental.
    
    If she has 3 identical cycles of over 90 days, neither longer nor shorter, it is revealed that the skipping of cycles was not a cessation of bleeding but was rather a change in cycle.  Therefore, the first period she had joins with the last 3 periods to obtain 4 periods and 3 intervals of 90 days.  But, if any of the cycles was either longer or shorter and therefore not equal, it is impossible for the first period to join with subsequent periods.  Until she has 4 equal periods (3 intervals) she does not establish an interval-based cycle. 

What's Going On - Young Girls

In modern times, the situation described in the seif is not terribly relevant per se.  However, it does form the basis of halacha that is applicable to menopausal women and is therefore important to review.

A girl of the age described is deemed to be immature until ascertained otherwise.  In halachic terms. she has a chazaka that she does not menstruate. The concept of chazaka in this context means that we assume the world continues today as it was before.  Prior to now, the girl had no menstrual flows; until proven otherwise the halacha assumes that she continues to have no menstrual bleeding.

The rule of 90 days is iron-clad - although there is some discussion in the poskim as to whether 3 onos beinonios actually means 90 days or if it means counting in the manner that we use to count - the last day of the previous cycle being the first day of the new cycle.  In that case, the time period needed for negation of her mature status would be 87 days.  Therefore, if she had been having 20 day cycles, missing three of them would cover 60 days, but a break of that length does not indicate immaturity.  So 90 days for this girl would represent 4 1/2 cycles.  On the other hand , a girl with 35 day cycles would hit the 90 days in approximately 2 1/2 cycles.

An adult woman, as we have learned, who misses 3 periods (based on her personal cycle, which could then total more or fewer than 90 days) no longer needs to suspect any day for her vesses.  However, once she has another period she returns to counting on her old cycle.

There are therefore 2 major differences in the way halacha treats an adult woman and a young girl.  First, our determination of cessation of periods is on a 3 period cycle for a woman and a 90 (or 87) day cycle for a girl.  Second, resumption of flows after that restores the old cycle to the woman and ignored the old cycle for the girl.

As a practical matter, the Badei HaShulchan points out that we assume any girl who is 12 years and a day has reached physical maturity as well.  He notes that we do not examine girls nor do we have the knowledge to affirmatively identify what hair growths qualify for consideration and what ones don't.

Finally, the Mechaber tells us that it is possible under very specific circumstances for a young girl to establish a cycle that is 90 days or more, if she has equally spaced periods of over 90 days that conform to the rules for establishing a fixed-interval cycle (vesses haflago kavua).