I feel for you so very much, regarding both your battle with depression and being in 2 minds about telling your older kids.
I can imagine it's an incredibly tough decision as to whether to tell your kids. For me, it was far less complicated. I left my years in depression behind me once my daughter turned 3 and my son was just a baby. My daughter will be 18 in a couple of weeks and my son is now 15. I told them about my struggles with mental health when they were in their early teens. Some details I held back on, for their sake. I had the benefit of clarity and good mental health so they were not so concerned about me, wondering how bad things were going to get.
I found there were many benefits to telling my kids. First, I wanted them to know life can get unbearable if we're not careful, if we don't have supporters in our life, people to consciously raise us. I wanted them to know that I understand how lonely and painful things may become for them when challenges threaten to overwhelm them. I wanted them to know their mum is a sensitive person, which includes being sensitive to any help they may need at any point in time. Mental health has always been of enormous importance in our household because we are open about it. When they've been down, they have felt the freedom of being able to cry openly with me because I am sensitive and can relate to mental torture. When they have been worried about the challenges they felt they could not face alone, they have come to me knowing how much I value the mantra 'We raise each other, no matter what'.
I can honestly say Danno that I have taught them well for they have raised me through moments of self doubt, though occasional moments where things felt like they were falling apart, throughout moments where I could not identify the challenge I faced. They have inspired my in ways that would have anyone believing they are young sages. Most kids have an amazing natural intelligence.
I have taught both my son and daughter 'real men' vent: They cry, they scream, they feel overwhelmed and lost at times, just like real human beings do. Bottle it up and feel the dis-ease (aka real men don't cry) is no way to live, in my opinion. I believe our kids need to know this.
There is always a chance that if you speak with your older kids, they may actually end up being the people who raise you in the most amazing unique ways. They may relate to you at times as the man who truly understands what struggle feels like.