A: This is a little difficult to put down an exact number of years. However, there where sentient beings on Asgard at least for millions of years (the Drakes). The other races the exact time is rather muddy and unknown. Much like it is here on Earth. The Aesir have been around for a long long time.
"Dulce bellum inexpertis."
Official Forum Expert on Norse Mythology
As Odin says in the Hovamal:
"Praise no day 'til evening; no wife 'til on her pyre; no sword 'til tested;
no maid 'til bedded; no ice 'til crossed;
no ale 'til drunk."