The sun sets below the mountain each evening. When the sun sets, a day has passed, so this means it is talking about time and age.
Oh my goodness i am 100 years in age (ps, im dead)
The sun sets behind the mountain every night. Every time the sun sets, you have lived for another day. This symbolizes the passing of time and age.
When the moon sets below the mountain. As you get older and older and your time goes past.
As the sun goes past the mountain and time goes by your age changes
The sun sets behind the mountain
The year has passed when the moon sets below the mountain