If someone feels really haunted by something they’re aware of doing as a very young child, before they have reached what the Church considers the ‘age of reason’ (i.e. somewhere between 5 and 7), can they bring it up in confession to receive absolution just in case they were culpable?
Yes, of course. You might explain to the priest that this was something that happened when you were 5 or 6 years old.
Well, there are actually two answers to this question.
The first is no. By definition, one cannot commit a sin before the age of reason. It is logically impossible. There is no sin to confess and there is no sin from which to be absolved & forgiven.
The second answer is maybe. If the act was truly a sin, then it can be confessed. That’s important; one would first have to determine that it was indeed a sin. If the penitent had attained the faculty to reason (even before the 7th birthday) before the act was comitted, then it was a sin and can (perhaps should) be confessed. This applies ONLY if that faculty to reason was present at the time (so the first paragraph still applies.)
To put it another way:
If you’re asking about “age of reason” then the answer is always no.
If you’re asking about the 7th birthday, then the answer is maybe.
If something is weighing on your soul then I think you can confess it. The priest will either reassure you that it wasn’t a sin or will be able to offer forgiveness. Either way you will have peace of mind.
And I do hope the OP finds peace. Don’t obsess about things you did as a young child, that’s not healthy. But if there is some particular thing then deal with it and forget it.