From Leona's judgment we know that the Rakkor participate in the Rite of Kor on the solstice after they turn 16. It also tells us it has been almost half a sun cycle since her Rite of Kor. On Earth, a solar cycle is about 11 years long. Assuming Runeterra has the same sun cycle as Earth, that would mean she joined the League about 5 years after turning 16. She was released July 2011, so it's been a year and a half since she joined. So assuming the sun is the same on Earth and Runeterra, Leona would be about 22, almost 23. Pantheon participated in the Rite of Kor on the same solstice as Leona, so he was also 16 then, and must now be 22.

tl;dr - If sun cycles are as long on Runeterra as they are on Earth, both Pantheon and Leona should be 22.

If you look close upon the Swain ingame Model, you notice quite a few things. One: His left side, if looked alone, is young looking. Not in the teens, mind you ,but closer to 30-40 than to 50. His hair has no signs of graying, despite the amount of stress he must have been through, all of his life, and his hair style deems that it is by choice. His right side, however, is vastly aged. Scarred, marked, and yes, shaved bald. This is also his bad leg, and the appearance of a cane tends to age a person, in our eyes.

Since rats only live 2-3 years, I'd say Twitch could have been anywhere from half a year to 3 years in age when he was mutated.

The mutation probably increased his lifespan though, but I'd say he's probably around 2-3 because he went to join the League very soon after he became intelligent.

Bracken's are immortal.

EDIT: Also, Galio is a lot older than 100. He stood over his master's corpse for 150 years before Poppy came and slept under him.

Immortal, that is. By the way, I do not consider Nocturne immortal, as he is the manifestation of nightmares. And while nightmares will never dissapear, they will lose the power to inspire fear into people. By that logic, while he IS immortal as we know, he, as a manifestation of something bigger, will lose and gain power accordingly the the thing he is a manifestation of, meaning that for example, people were to never sleep or dream again, he'd fade and die, so he is not wholly and truely immortal.

If ''hidden'' lore is to be trusted, Swain is at least as old as Emilia.

Wukong 2-3 years old, now thats weird

skarner has lived for thousands of years, it says in his lore that in one of the first rune wars his race attached themselves to "life crystals" and then went into underground hibernation before being woken by the magical conflict at the place where dominion is.