It's pretty interesting stuff. Here's why the age of the galaxy is important:
We must first explain how galaxies are categorized by color. Though it may seem counter to the colors we usually associate with hot and cold, young stars, which tend to be hot, are blue-colored, while old stars, which tend to be cooler, are red-colored.There's more on why our galaxy is a suitable habitation for living things at the link. The sorts of things Ross says about the Milky Way can also be said about the solar system and the earth/moon complex. When all the unique factors which have to be pretty much just as they are for higher life forms to be sustained anywhere in the cosmos are tallied up the improbability of it all has led some scientists to conclude that it's very unlikely that there's any other place in the universe where life like ours could exist.
So galaxies in which star formation proceeds aggressively shine with a blue color, while galaxies in which star formation has ceased appear red.... Astronomers have typically categorized galaxies as belonging to either the red population or the blue population.
The Milky Way (MWG), however, fits into neither the red nor the blue category. It has taken on a green hue. This is because, while star formation in the MWG has subsided some, it has not yet ceased. Thus, our galaxy contains a combination of blue stars and stars that aren't yet old enough to be red but have aged enough to be yellow. Blended together, these stars give the galaxy a green appearance.
Green galaxies are rare, but they are exactly what advanced life requires. A galaxy dominated by blue stars will bathe its planets with many flares—flares too abundant and intense, and with too much ultraviolet and x-ray radiation, to permit life to exist on any of the planets.
Diagram of the Milky Way Galaxy showing the location of our sun
A galaxy dominated by red stars will also bathe its planets with many flares—again, flares of deadly intensity. A red galaxy also exposes its planets to more supernova and nova events (stellar explosions) than advanced life can possibly handle.
Another problem for galaxies dominated by red stars is that they lack the necessary level of ongoing star formation to sustain their spiral structure. But galaxies dominated by blue stars, where star formation is advancing aggressively, experience major disturbances (warps, bends, spurs, and feathers) in their spiral structure, so they cannot maintain a stable spiral form either.
But the green Milky Way, in addition to being of appropriate size and mass to contain the elements that life requires, has another characteristic that allows for the existence of advanced life within it: its spiral arms are stable, well-separated, highly symmetrical, free of any significant warps or bends, and relatively free of spurs and feathers.
In part, these spiral-arm features are possible because the galaxy is dominated by yellow stars which are complemented by a significant population of blue stars.
[O]ur galaxy....is transitioning from a star-forming site to a no-longer-star-forming site. And this midlife period appears to be the "best of times" for the sustainment of living things....[T]he Milky Way has transitioned from its role in building the required ingredients for advanced life (carbon, nitrogen, oxygen, phosphorus, calcium, iron, etc.) to one in which it can now, for a relatively brief time period, sustain advanced life.
One could perhaps say that the existence of another habitable galaxy somewhere out there, with a solar system and a planet capable of sustaining life, would almost be miraculous.
