Property law validating life
Because the interest will not vest until her new child reaches 25 years of age, which cannot happen until more than 21 years after A and her other children (together who form the "lives in being" to which the rule refers) have all died, the rule against perpetuities makes the entire gift "to the first of A's children to reach 25 years of age" void.A will hold Blackacre for life, and then the property will revert to the person whose will transferred it to A in the first place.The fertile octogenarian is a fictitious character that comes up when applying the rule against perpetuities.The rule presumes that anyone, even an octogenarian (i.e., someone between 80 and 90 years of age) can parent a child, regardless of sex or health.
On the contrary, the class of "A's issue" is subject to expand long after A's death, so their future interest remains that of a vested remainder subject to open, which is within the scope of the rule.
If B's new wife were to outlive him (making her his widow) and survive him by more than 21 years, then the interest to "B's children then living" would not vest until after the perpetuities period expired (21 years after the death of B, the only relevant life in being at the time of the devise), because only upon the death of the widow can one ascertain who are "B's children then living." Alternately, if B is not married at the time of the devise and B were to get married afterwards, again the wife could not be a life in being since she is not identifiable at the time of the devise.