Reason for the death of Bhishma
Bhishma was very noble person and living according to Dharma. He was also the son of Ganga and was an immortal as he had a boon of dying only when he wanted to. He was also the strongest warrior in Mahabharata war (even stronger than Arjuna and Karna).
Facing such death is really surprising, but it was related to his past karma.
While laying on bed of arrows, Bhishma asked the same question to Krishna as why I am facing such death. Krishna enlightened him with all his previous births. In previous 75 births he was always born as Prince and lived a noble, kind person and lived according to dharma. But in last 75th life when was a teen, he was playing with bow and arrow. While playing, he saw a lizard and hit with arrow for having fun. The lizard fell into bush of thrones and died after suffering.
His previous lives of good karma saved him for this fruit of bad karma, but he got the fruit of when his soul incarnation as Bhishma and he was standing against Dharma.
Age of Satyavati when she leaves Hastinapur
We can only guess. Let’s say her age was 25, and her husband Shantanu’s first son Bhishma was also about the same age. So let’s say 25 years later Vichitravirya dies sometime after ascending the throne and marriage. Then let’s say Dhritarashtra, Pandu and Vidura are born a year later. So now Satyavati is 51. 25-30(let’s assume 30) years later, they grow up and Pandu ascends the throne, and knowing that he expanded the Kuru empire a lot and became a Chakravarthy, it would taken at least 2–3 years for that. So now, Satyavati is around 80 years old.
Within a year or two, Pandu marries Kunti and Madri, and goes into exile. Satyavati is 84–85. Pandu becomes a father to Yudhishthira, and sequentially (1 year gap between everyone) to all the 5. The Pandavas returned to Hastinapur when Yudhishthira was 16, so Satyavati is 100-101.
When she gets to know that the able king, Pandu, has died, and she is warned by Vyasa that there will be a lot of tension and destruction in the future for the Kuru clan, then she renounces the kingdom to live in the forests with Vyasa (who is also her son and the writer of Mahabharata) with Ambika and Ambalika. So after that we don’t know about her, so she is around 100 years old when she leaves Hastinapur.