I have decided to group the Eternals in Generations based on significant events in their history, such as the Celestial Hosts. In order to understand this better, I will first give a brief outline of each Host.

The First Host describes the first visit of the enigmatic Celestials to the Earth in it's prehistoric past, about 1 Million years ago. These cosmic beings performed experiments on man's ancestors, creating both the Eternals and Deviants. "The First came to Earth before there were men upon it...but they did find our common ancestor, who became subject to their experiments." (Ajak- Eternals 7).

The Second Host describes the Celestials second visit to the Earth, around 20 000 years ago. The Deviants had created a world-wide empire, and thought they could challenge the Space Gods...they were wrong. "Wrath and Discipline marked the coming of the Second Host! Many civilizations spawned in the dim past vanished, forcing man to climb again in new directions." (Ajak- Eternals 7).

The Third Host describes the third visit of the Celestials, around 3000 years ago. "Inspection and Cultivation was the job of the Third Host. The Inca civilization received them as Gods. The Celestials roamed the world on strange vehicles, and caused consternation among the superstitious." (Ajak- Eternals 7).

The Fourth Host arrived much more recently, only a few years ago. The Celestials usually only visit a planet four times, and the fourth time is a visit of judgement, either in the planet's favour, or against it. Thankfully, the Space Gods voted in Earth's favour, and departed.

Makkari also once commented that Eternals celebrate their birthdays once a millennium- making Makkari 4 & 1/2 at present.  And that the original OHOTMU claims that an Eternal does not reach adulthood until they are 1500 years old.
Here then, is my interpretation of the Eternals Generations.