With my teacher head on I'd say that 8 is about the age that I'd be thinking about some one to one work to get it sorted, if that hasn't already happended.
If it's a specific confusion (some children just have a bit of a blind spot when it comes to some letters and numbers) I wouldn't be 'worried', but if it was part of a general problem with spelling, such as serious difficulty in recognising simple phonic patterns, I would be suggesting some SEN time.
I'm not intending to panic anyone, I'm just a believer in children getting those little bits of support when they need them and earlier rather than later for preference.
(Retreats under stone to drink wine )