Id say if it's affecting your daily life then it's something you should talk to a therapist about. By that I mean if your depressive times are stopping you from doing things you'd normally do - go to work, get out of bed, etc. - then it's something you want to look into.
If it's just the mid life" Why do I even bother some days/why do I exist" then I'd say it's normal. Not pleasant, but a normal part of life to feel down at times.