As far as I'm aware the idea that the A-wings were a new advanced design is an EU invention based purely on the observation that they didn't appear on Yavin or Hoth. There's no reason why they can't have been around a while, even if it's just a few years prior to ANH. Personally I rather like the idea that their design looks rather like a true successor to the old Jedi Starfighters, while the TIEs are a cheep, no frills knock-off.
The same could be said of the X-wings since they never looked like they were supposed to be as brand spanking new as the EU made them out to be. There's no reason why they couldn't have been around in the last days of the Clone Wars.
If you go by hardcore canon, i.e. only the official movies and nowadays TV shows (of which Rebels is the only one that is current canon) there is little if no explanation so you can speculate all you want.
However old EU clearly stated all this across various sources and it said that the X-Wing is a rather new design that was a contender for the new main fighter for the Empire but they went with the TIE and the engineers stole the plans for the XWing when they defected to the Rebellion.
As to their looks it's easily explainable because of the Rebellion's low ressources. Keeping them in visual top condition wouldn't exactly be a top priority.
The A-Wing is an even newer development by Jan Dodonna post Battle of Yavin.