Os-Gabella is a true immortal, unable to die or even physically age so long as the world exists. She is older than anyone but the gods themselves, older even than the archangels. I don't know how long this is, but it could be millions of years.
Sabathiel, Cassiel, Basium, and Hyborem are ancient, dating back to the godswar. They are at least a few thousand years old. Hyborem is the youngest archangel, as he was made from a corrupted reflection of Cernunnos late in the war. Cassiel and Basium have fallen, and so are now mortal at least in the sense that they can be killed. I don't think they age are are prone to any diseases though.
Man was meant to be immortal, but thanks to Gabella's rebellion they are descended from the soulless, mortal second wife of Nemed and have inherited her physical mortality along with his spiritual immortality. The first generations of man were extremely long lived, perhaps even living tens of thousands of years. Their lifespans have gradually diminished over time, and are now no longer than they are in the real world. I suspect that in the Age of Magic men may still have lived a few centuries on average (at least those of the upper classes), but this dramatically decreased in the Age of Ice. Kylorin himself was given eternal youth by Ceridwen, so he is essentially immortal.
The Elves are descended from men who in the Age of Dragons came to be very close to Sucellus, then God of Nature. They seem to have kept the longevity of early man better than men have. They seem to be essentially immortal much like the archangels, but they are no harder to kill violently than a human and are quite vulnerable to diseases. They tend to have batter sanitation than humans, and so don't have the same immunities so a simple cold or flu is often lethal if medicine is unavailable.
Faeryl Viconia and Arendel Phaedra were both alive in the Age of Magic, and may even date back to late in the Age of Dragons. They are thus millennia old. Yvain is as old as they are, but he was retconned into being a Treant instead of an Elf because Kael wanted to borrow some warhammer graphics.
The Aifons were humans that grew close to Danalin, and become amphibious. I suspect their lifespans were close to that of the Elves, and that the two races were rather close. However, they are all dead now, and no one even really knows where their souls went. This is a great mystery associated with Danalin choosing to go to sleep, and with the creation of the Overlords.
Perpentach's body isn't that old, because he transfers his mind to a new one whenever it gets old. His mind dates back to the Age of Magic, and is likely around thousand years old.
Capria was born late in the Age of Magic, and spent her childhood fighting her way out of hell, where her body was but a manifestation of her mind and spirit and so did not age except through growing more emotionally mature. She would have physically been in her late teens when she emerged around 400 years later, but has all the experience of one with such a long life. She should have stared aging naturally and at a normal rate after that, yet she still seems relatively young at a point that should probably be about 300 years later. This seems like it may be an inconsistency, but maybe 300 years old is only middle aged for someone from the undiminished royal line of the pre-fall Bannor.
The Orks probably live about as long as modern humans, or possibly as long as men in the Age of Magic. They are descended from those who fell with Bhall at the cusp between the Ages of Magic and Ice, mostly from the Bannor. In Kael's D&D game, Orthus was friends with the Bannor king of Braduk before the fall. Orks apparently look pretty much human when they are young, and grow more monstrous with age. Sheelba is still quite young.
The Dwarves are said to have been crafted by Keldon Ki in the Age of Magic, and given life by Kilmorph. (I have my doubts about this, as it seems to imply that they have no souls and there is evidence that they do have them. I tend to think this may be a myth or possibly only an allegory that humans have interpreted as being their sincere belief. They may just men men who like the elves grew close to their god and were changed.) It is said that the first generation of Dwarves (which include Bambur) are immortal, but that their lifespans get shorter with each generation. This is a lot like humanity, but their lifespans seem to be shrinking faster. I suspect that dwarves still live a little longer than men, but that won't last long.
I tend to think that the Khazad rulers may be close to around 120 years old, and considered middle aged. Their grandchildren probably won't live that long though.
The Luchuirp leaders seem younger, in fact I think Beeri might have still been considered a child (which may mean teens or 20s) when he was captured by the Baleraphs. Garrim would be a little older.
Ethne was fairly young, probably in her late 20s or early 30s.
Einion is considered ancient, but apparently he was born in the Age of Rebirth. He may be close to a hundred years old. I would rather think he was actually born in the last years of the Age of Magic to a noble Elohim family and is thus around 500 years old, since that would mean he has personally seen his people loose their holy sites and should have a greater desire to get them back.
I picture Decius as a man in his 30s, or possibly late 20s when he first appears in the scenarios and perhaps his 40s by the end. He is from one of the most noble Bannor lineages, so his father who is probably at least 80 is still strong enough to lead his forces into combat.
Rhoanna and Tasunke would be in their late teens when they first came to power, but likely in their 30s by the time of many of the important events that happen to them.
Cardith Lorda is the youngest ruler, but he is not as young as he appears. He stopped physically aging when Eurabatres found and merged with him, around 8 years old, and has wisdom beyond his years. He is essentially immortal, but may begin aging like normal when Eurabatres gets a body of his own.
Varn Gossam is probably at least 500 years old. He was either born in the underworld or kidnapped by Laroth with his parents while still quite young. That would have probably been late in the Age of Magic, after Kylorin killed Laroth but before the Fire Rain. He probably spent at least 300 years in the Shadowed Vale as a prince and subject of his brother Haerlond before meeting Auric, and had wandered the surface for years before finding the Mirror of Heaven and founding the Empyrean. He and the other Once Elves knew nothing of the Age of Ice, the Elven Civil War, or even of the Summer and Winter Queens.
Auric was around 16 year old when he met Varn Gossam, but probably at least 20 when he began to lead the Illians as the reincarnation of their god. By the time of his death, he has either aged to be at least in his 70s or the stress of his ascension took its toll on him and made him seem much older than he really was. When his soul reaches the underworld it is the spirit of a tired old man.
Valledia and Dain are probably both middle ages, with Dain being a little older. Dain has decades of experience in magic and Valledia in politics and subterfuge.
Keeyln physically matures into her twenties by the end of the tale, but mentally stays around age 6.
Hannah left the ship of her adopted father at age 15 and soon led a mutiny against her next captain. I picture her as being a powerful despot of the largest ever unified fleet of Lanun pirates before she turns 20. She is a chosen one of the overlords, so the sea itself may seem to bend to her will. No one likes her, but they are too afraid to complain.
I tend to think that Falamar probably became renowned for his bravery and skill with the ladies while still a teenager, and was likely elected captain in his early 20s without having to use any coercion to gain the position. He was likely close to 30 by the events events of the scenarios.
Tebryn Arbandi is a dead patrian archmage, who has been brought back and will stay alive so long as he remains in Ceridwen's good graces. He has inscribed a resurrection rune of Abashi's forehead, so he cannot die until someone dispels that, which for anyone but Ceridwen would require slaying the dragon.