Growing up and Growing Old in the Realm

Humans, Dwarves, Giants, and even the long-lived Day and Night Elves are considered mortal beings. Gods, Vanir and the Elementals are immortal, but that doesn't mean they all live forever.
Gods are nearly as squishy and vulnerable to disease, poison, accident and murder as humans, but if they manage to avoid dying, they can techinically live forever.
Vanir don't have to worry about poison or most disease, but the rest still applies, and being exponentially less social than the Gods as a rule, their population has remained sparse to moderate.
The Elementals don't "die" so much as they melt or evaporate or cool to petrified rock, shatter to ash under the wrong conditions, by their own doing (purposeful or accidental) or by outside forces (sentient or natural). Their methods of reproduction are all over the map compared to the rest of the Prime species in the Realm, in both method and frequency of occurence - anything from the humanoid ways to the fertilization of caches of eggs to the combining of pieces or essences to parthenogenesis.
A life in Beardsgaard starts with youth, childhood and the growing years, not always under a parent's care for all of it (Vanir tend to be raised by the woods from ages 10 to 100). Even with a wide variety of lifespans among species, maturity is more cultural than biological.
Immortal Gods (and Humans) certainly think of themselves as adults at 16 (even if their parents do not), and they are adult sized. Giants hold the view that you are not done cooking in your home hearth until 1/5 of your life has elapsed - around 100 years. Most of the immortals share this idea with the giants. Short of the Gods, immortals think 100 years is about the right amount of time to learn your way in the world before being set loose on it.
Adulthood is when some grow their families, and some grow their experience in the world before the middle years that are held in deep regard as a time to plant roots and grow, in preparation for one's elder years, for family or for legacy.