To tell or not to tell, what age do you tell your kids about your depression?

I can't tell you how bad I feel at the moment,  I feel I am a bad Mother.  It has come to our attention that my Son is behind with Maths and it's my fault.  It seems that as I have been battling with pain and depression that my Son has become another victim of this cruel disorder.  I knew this would happen I could see it coming, I did try to avoid it but I failed.

I have been getting my Son tutored for years now as I have not had the strength or brains ( My brain simply refuses to work now ) to do it myself,  in saying that I do help to a certain degree.

I thought I was doing the right thing, not wanting him to slip through the cracks.  We stopped getting him tutored last year as we thought all was well, but that's far from the fact.  I now have to pick up the pieces and get him up to speed.

I feel I owe him an explanation, otherwise he might think I just don't care.

What is a good age to tell your children that you have depression???   I was always going to tell him at some stage, because that's how you stop the stigma that has attach itself to the label we are given.

But I'm scared it will affect him in some way, maybe with self blame or I don't know what else but something.   I'm not embarrassed ( maybe a little ) but it's the fear of how he will take it.

I only have one Son, how did I screw things up?, I thought I had it covered.

What will I do????