I made both using size 7/8 Flashback pattern (i.e. the “big kids” version of the pattern – it comes in two size ranges), even though he’s nine years old. It fits him perfectly right now, but I cut a new t-shirt from the next size up to make a summer t-shirt for him, so by next fall it might be too small.
That's one of the things I like about the Flashback model is that it seems to last a long time; it looks a little oversized when they first wear it, then it turns into a real “skinny t-shirt” until it hits mid-arm and you have to sneak it out from their drawer while they are not paying attention. There might still be one or two size 6 Flashbacks in his dresser right now that he's still wearing. Hmm.