The 'age of accountability' is an attempt to articulate that children shouldn't suffer the full consequences of their sin because they are not fully aware of its significance. I tend to agree that it's not a biblical doctrine, but I'm not sure what the alternative is. It is possible to construct an argument for parental responsibility, not least given the way in which judgement of adults also catches the children in the OT. The other way forward appears to be a suggestion that they are held accountable for their own response to God's grace, which is equally iffy. Or we can just give up and say 'We don't know because we haven't been told' - which does have a significant attraction!

It's not a topic that gets much attention in the circles I move in, not least because we struggle to believe in Hell at all, so details about how it affects children is WAY off the agenda, so I'd be interested to hear your views about it.