Similar idiom to 'Do not feel sorry about the turn of the times'

Consider you can't stay young forever.

Example: While it's true you can't stay young forever, you can remain youthful in spirit

You may also quote "Le Cid" written by Pierre Corneille: "Oh rage, oh despair, oh vile old age!" (translated from French "O rage, ô désespoir, ô vieillesse ennemie")

 EDIT to address the "vice versa" issue:

Life is not a long quiet river.

You could consider using Life has its peaks and valleys. Peak means:

The point of highest activity, quality, or achievement: he was at his peak as a cricketer

[Oxford Online Dictionary]

And valley means:

a low point or interval in any process, representation, or situation.

[Dictionary.Reference.Com]

The word peak signifies at the top and valley at the bottom.

Basically, "Life has its peaks and valleys" means "Life has its ups and downs or ebbs and flows".