various events leading into Rebirth already altered the timeline (through events or single issues) and added previously missing events back into the timeline, which already expanded the timeline; a trend that continued even after Rebirth. like I said, they may have stated AT THE TIME that he was 10 but no matter how you shake it he would HAVE to be 12 by the time he gets to Rebirth because Infinite Frontier reaffirms Rebirth's birthday and which was in-canon only a few months after We are Robin/Robin War.
you are working against the obvious answer because you are working under the assumption that the older comic holds more or equivalent canonical weight than the newer ones; it's the other way around. they can state he's a literal infant at the time but if they retcon he was 22 and you look at the story and it conceivably looks like he could be 22 then thats the answer. you have to remember these ages truly don't matter that much, everyone looks more or less age-ambigious and abnormally fit. you simply go with the ones that make the most sense with the established canon. unfortunately these things get overcomplicated because the dominate school of thought is "lol comics are confusing" instead of just making it make.
Also it wouldn't make Duke 18, it would make him around 18. we only know for fact that Duke is a year older after training, which started a few months after he was stated to be 16. I'm fully aware Duke should be around 18 based on his timeline from WaR to BaTS to Outsiders to now, but the Multiverse resets leading into Infinite Frontier makes anything after the year declaration murky. he could be 18 or he could be turning 18 or older, but until DC gives him a birthday scene or does something that indicates he's of age, 17 is the safest assumption. DC doesn't touch on Duke's age too much, his last stated age was 16 in We are Robin (and even then it wasn't even in the actual story, it was in the companion character profiles), so for all we know DC could confirm he is in fact 17, 18 or even 19 at any point.
also, regardless of him being 18 or 17 Jay's treatment doesn't really indicat one way or the other. turning 18 doesn't make family suddenly treat you like your an adult