Is there a period when a marriage goes through a really rough patch?

Married for 16 years and things are rough right now. Appears that couples who have been married around this amount of time are falling apart all around me. Is it because kids are bigger and don't need the constant care and these problems were here to begin with just not paid attention to or is a mid life crisis... Is there anyone out there who has been married for a long time and can shed some light on this? Wondering if I talked to someone married forever if they can relate....