When my blood sugars were in the 20's all of the time I used to think I was going hypo at the 5-6 mark. However, now that my sugars have improved I start to get symptoms probably around the low 4's (4.0-4.5). My symptoms have always been very strong even as a child and when my sugars were good as a child they would always start at the same number as what it does now. I personally don't think there is a "healthy" level for symptoms to start as the sugar they start at can range for many many reasons and what is healthy for one might not be for another.