I'm not sure its a good idea to consider the childrens' ages individually, since they may be end up talking with each-other and learn they got a different story. I don't believe you should burden your eldest children with keeping such a secret; I would keep the story consistent for all of them, and not tell any of them anything you wouldn't want youngest to try and grapple with too early.
Mental illness that extreme can faithfully be considered an illness, so you aren't lying to them. I don't see any reason they have to be told all the details, all the important things you want to convey to them should be enough for them to think about, i.e.
You can still tell them that an illness killed their uncle, without opening the Pandora's box that is suicide to them. As they grow older they may even realize themselves from these details what happened, which assures you that if they understand suicide enough to deduce it, they are ready to deal with the specific truths of this situation. Just be ready to answer their questions down the road.
I think the most important thing you can do is let them know how depression may affect them, how to identify it, and that they have support to deal with it. You can do this without laying any existential burdens on them by way of telling them too much.
If you still personally have a strong urge to tell them everything, it may be because you have a very natural urge to get this off your chest, but your kids should not be responsible for this. If you really need to talk to someone about it, open up to another adult in your life who you trust or seek professional therapy, then reconsider what you believe is important to tell your kids.