Age differences
Karna: said to be around 16 years older than Yudhisthir (could be 10 but 16 is the more common notion)
Yudhisthir: 1-3 years older than Bhima depending upon interpretation/source
Bhima: one year older than Arjun (some say he was 2 years older)
Arjun: one year older than Nakul and Sahadev
Krishna: same age as Arjun
Balram: about 1 year older than Krishna (some say he was even older)
Gandhari (mother of the Kauravas) had a very unusual pregnancy which lasted for 2 years. At the time Yudhisthir was born she was already pregnant for a year (or 2 years depending upon the interpretation/source). Eventually she grew frustrated and beat her womb causing herself to give birth to a ball of hard flesh. Sage Vyas divided it into 101 parts and put them into pots full of ghee. Two (or one) years later the Kauravas were officially born in those pots.
Duryodhan was supposedly born either on the same day as Bhim or the day after. So, if Yudhisthir was born at the end of Gandhari's first year of pregnancy then it would be 2-3 more years till Duryodhan's birth; this would make Yudhisthir 2-3 years older than Bhim/Duryodhan. If Yudhisthir was born at the end of Gandhari's second year of pregnancy then he would have been around 1-2 years older than Bhim/Duryodhan.
The Kauravas were all born one after another, one day apart from each other (ie Duryodhan on the first day, then Duhshasan on the second day and so on). The daughter Dussala was last. Hence from the birth of Duryodhan till the birth of Dussala a total of 101 days passed.
Arjun was born one year after Bhim. This would make him around 9 months younger than the youngest Kaurava. Some say Arjun was 2 years younger than Bhim making him almost 2 years younger than the youngest Kaurava.
Yuyutsu's birth is a bit more confusing. He was Dhritrashtra's son born of a maidservant. Some sources say he was born on the same day as Duryodhan but after him thus making him slightly younger than Duryodhan but older than the remaining Kauravas. There is a verse that says he was born during the last phases of Gandhari's pregnancy making him about 1-2 years or more older than Duryodhan and possibly older than Yudhisthir (if we assume Yudhisthir was born at the end of Gandhari's 2 year pregnancy) or less than a year younger than Yudhisthir (if we assume Yudhisthir was born in Gandhari's first year of pregnancy). Another verse says that he was born after the births of Dhritrashtra's children, making him younger than them all and about 9 months older than Arjun.
Age during Kurukshetra War:
Yudhisthir: 91 (actually 91 + couple of months)
Karna: 107
Bhima: between 88 and 90
Arjun/Krishna: between 87 and 89 (using Bhima's age above)
Nakul/Sahadev: between 86 and 88
Duryodhan: same as Bhima, so between 88 and 90
Yuyutsu: some possibilities - (a) 91 or slightly less, (b) 90, (c) same age as Duryodhan, (d) 100 days younger than Duryodhan (see above)
Bhishma is the big conundrum. Some say he was several hundred years old when he died at the end of the war. But this is difficult to sync with the ages of the Pandavas and Kauravas. Depending upon interpretation, Bhishma was probably around 60 years old at the time of Yudhisthir's birth. This would make him over 150 years old at the time of the War.
After the War:
Dhritrashtra. Gandhari, Kunti and Vidur died about 15 years later.
Krishna was said to be 125 years old at the time of his maha samadhi which happened about 36 years after the war. Yudhisthir would have been about 127-129. After this event, all the Pandavas crowned Parikshit king and left Hastinapur. They performed pilgrimage to many places before finally returning to the Himalayas where they perished (except Yudhisthir who ascended to heaven alive). The exact time between Krishna's samadhi and the Pandavas' end is not clear. Perhaps a decade or two. 🤔
Once again, keep in mind there are many other ways to interpret the text and compute the dates and ages. 😊
Also, don't be surprised at the characters being so "old" (by modern standards) at the time of the War. According to mythology, people had a much longer lifespan back then and each phase of life (childhood, young adult, middle age, old age) lasted for longer than they do today. 😉