age - The "Miracle" of Hagar's giving birth

This question, coupled with some math I did today leads me to a question.

Yishmael was born (according to the Jewish Timeline Encyclopedia) in the year 2034 from creation, the same year that Avraham married Hagar. He was circumcised at age 13 (2048) and Yitzchak was born 6 months later. He and Hagar were sent away in 2061 (at age 27). The Akeida was when Yitzchak was 37, in the year 2084. At this point (which leads right into Sarah's death) 50 years had passed since Avraham first married Hagar.

Avraham (re)marries Keturah/Hagar (cf) either in 2088 or 2096, ranging from 54 to 62 years since she (assuming Keturah was Hagar) birthed Yishmael. I don't want to assume that Hagar was married to Avraham when she was under a reasonable age (say 12) regardless of opinions that Rivkah was 3. So using the age of 12 as a minimum, Hagar would have to be between 66 and 74 years old when she started giving birth to the next 6 kids. Sarah knew that (at age 90, based on a death at 127 years old in 2084 and Yitzchak's birth about 37 years earlier) she would not conceive naturally because her cycle had stopped already.

Unless one assumes that at that time, women continued their menses until well into their 80's (which would make the miracle of a 90 year old birth less amazing) it seems strange that Hagar would be able to conceive and carry children naturally so late in life. I can't figure out not why the Torah wouldn't ascribe any miraculous nature to the ability of a clearly post menopausal woman to give birth, but why Hagar would deserve to be the recipient of such a miracle.

[Yes, I know I have made a few assumptions about Hagar's initial age and the time of the cessation of the menstrual cycle at that era, but if someone can give me facts and sources which show me the numbers or facts are different I'd appreciate that.]

Answer

The Ramban addresses this in his commentary to B'reishis 17:17. He explains that miracle of Yitzchok's birth was not the age of Avraham or Sarah (as Avraham later had children from Keturah (Hagar) when he was 140). At that time, as long as people remained fertile they could still conceive children past age hundred. Rather, the miracle was in the fact that this couple who were childless for so long could bear children.

The Lubavitcher Rebbe also addreses the question (Likketey Sichos Chelek 20 pg. 81) and explains that it was specifically Sarah who was barren and miraculously gave birth. Avraham (and Hagar) were never barren and thus having a child was natural.