Probably Trying To Hard

Iruma is the only character to have an explicitly stated age. Starting at 14 and turning 15 during the series.

We learn that most demons only celebrate their birthday once typically when they're younger. It's never stated during Iruma's celebration what age he's turning.

To the other misfits he's just having a birthday as a "teenager".

We know that demons can hundreds of hears even going so high as even over a thousand years. Grandpa Sullivan is implied to be over one thousand years old given that's how long The Demon King has been missing and he served The Demon King when in his, Sulivan's, relative thirties or forties judging by appearances. Some other characters are shown or otherwise stated to be around the same age as Sulivan such as the other two Greats, and Derkila's close friend Amduscias. Levi of the Three Greats is accused by the other two of using magic to keep herself looking young, one can only assume Amduscias is doing something similar. This leaves us with three to four demons who are over one thousand but look and act to be roughly in their sixties.

Kalego and the rest of the teaching staff typically look to be in their thirties and forties, with some of the younger ones looking to be in their twenties like Robin. Personally I've always seen Kalego to be around 33 to 35 in appearance.

We've never gotten any solid hits to how old he is in years, but I've always assumed that he was at least over 100, and this gets messy with what we've learned over the last 5-10 chapters.

To refresh some surrounding context, Kalego when he was going to Babyls as a student was in the same year as Shichiro (yes I'm on a first name bases with him), and in the year directly below Opera.

Now, in the continuing flashback centering on Ameri's father Henri we've learned more about his past and Ameri's mother, how they met and how she came to be born.

During the protracted view into the past we've seen and met many familiar faces. These familiar faces, as of my writing this, complicate how we view aging in the netherworld.

Henri in the present, only looks to be in his forties, a reasonable age to have a daughter in her mid to late teens, by human standards. This would place, visually only five or so years older than Kalego.

BUT in the past, while Henri along with future professor Raim and Sabro's father, appears to be in his early twenties, we meet Kalego Opera and Shichiro all as children looking around five years old to my eyes. This is important while we don't get a certain timeframe for the courtship of Ameri's parents, her birth is implied, or so I understand it, to take place not long after they met. Even say Aermi is born two years after her parents met, that would make Kalego Shichiro and Opera all only seven years older than Ameri. But in the present time, Ameri look around 17, and acts as such, while the three above adults all look and act to be in their thirties.

I have privately speculated previously that the way demons age is that human age normally like humans but only up to a certain age after which the rate at which they physically age drastically decreases but doesn't stop. For example one would age normally until say thirty and then they would age very slowly from then onward where centuries replace decades in an unequal balance.

This might explain the ages and apparent ages of some of the older demons in the series, and would assuage some of the complexity or conflict some people feel about Iruma entering a relationship with any demons in his orbit. His LIs are all the ages they appear to be.

The only problem is wee baby Ameri.

So innocent, so cute, so damn fucking complicated.