This is my first time responding here!
From what I've found, in Thor, when speaking to Odin, he clarifies that they are not gods, nor immortal, to which Loki responds "Give or take 5,000 years".
If we do some math and say 5,000 is the average life expectancy of a typical Asgardian (according to Loki), and divide that by 80 years here on earth, we get 62.5.
According to Norse Mythology, Thor should have been a few hundred years old in 965AD during the battle with the Frost Giants to conquer earth. Loki was found/taken after the war, as seen in the first Thor movie.
Now, according to the official marvel timeline, 1,000 years before Tony Stark declared "I am Iron Man" in 2008, Odin took the Casket and Loki, making Loki's birth year around 1008.
2018-1008= 1010 Asgardian years old.
1010/62.5= 16.2 Earth years old.
Since Thor was a few hundred years old in 965AD, and 400 years after taking the casket and Loki, Odin left the Tesseract on Earth, that means that Thor would have been considered an "adult" by that time (1,408AD)
In Viking culture, a male was considered an adult at 10 years of age and able to fight.
We can calculate this 2 ways:
625/62.5= 10 years old.
625-400= Making Thor 225 years old in 965AD (Thor would have been only about 3 and a half during the war and obviously couldn't fight.)
965-225= Makes his birth year 740AD
2018-740= 1,278 Asgardian years old
1,278/62.5= 20.4 Earth years old.
1,408-625= Making his birth year 783AD
965-783= Making Thor 182 in 965AD (Thor would have been 3 years old and, again, couldn't fight)
2018-783= 1,235 Asgardian years old
1,235/62.5= 19.8 Earth years old
Either way, Thor is about 20 Earth years old, and Loki is about 16 Earth years old.
Please feel free to critique my math. It's nearly 2am by the time I post this.