What age should we tell him?

I have a 7 year old step son who I love just as much as my 2 month old son. I have been in his life since he turned 3 and his real mother is completely out of the picture. We have never lied to him and said I gave birth to him like I did his little brother. He calls me mom and doesn't really remember there ever being a time when I wasn't there. If he were to ask we would of course tell him but he hasn't. I have no problem telling him that I'm not his real mom because I know he will love me the same and know that I love him no matter what. The only thing I'm worried about is explaining to him why his mother is no longer around. I don't want him to think that it is his fault in any way because it's not. It's hers. She's the one who decided to leave when he was less than a year old and never visit(even though she lived within walking distance). She's the one that did drugs and never paid any attention to him. After a while my husband and I decided to get her rights taken away. I just don't want to do it too soon or too late. If he ever wanted to meet her I would of course let him but I don't want him to be hurt by her uncaring.