After death of Pandu, Vyasa advised Satyavati to retire into the forest and engage in the contemplation through Yoga.
On this advice, Satyavati retired to forest where she left her body:
Acquiescing in the words of Vyasa, Satyavati entered the inner apartments and addressed her daughter-in-law, saying, 'O Ambika, I hear that in consequence of the deeds of your grandsons, this Bharata dynasty and its subjects will perish. If thou permit, I would go to the forest with Kausalya, so grieved at the loss of her son.' O king, saying this the queen, taking the permission of Bhishma also, went to the forest. And arriving there with her two daughters-in-law, she became engaged in profound contemplation, and in good time leaving her body ascended to heaven.'
~Mahaabhaarata: Aadi Parva: Sambhava Parva: SECTION CXXVIII
But Satyavati was his mother, so, she must have been ~160 years old if she was alive when Bhishma died
It's not necessary that Satyavati's age has to be more than that of Bhishma as she was not biological mother of Bhishma. When she married to Santanu, Bhishma was already of matured age. So, she was more or less of the same age of Bhishma. As she died after Pandu's death when Pandvas were infants, so she would be in her sixties or seventies [ 140 (Bhishma's age in Mahabharata war) - 91 (Yudhisthira's age in Mahabharata war) + 16 (Yudhisthira's age when Pandu died) + Y (years she spent in forest) ].
What were the ages of Pandavas & other characters during the Mahabharata war?