Bingowings said:

I prefer to think of all the Rebel fighter ships as being pretty much as old as each other.

I've never bought into EU idea that the A-Wings were new.

I think it's more a matter of the Alliance growing as the Empire became more oppressive and the news of the Rebel's victory in the first film got passed around.

That would mean as more people joined they brought their own old ships with them (which would be pimped up by Rebel technicians).

The fleet massing for one last push would mean assets that had never been brought together before would come into play.

So Y-Wings would be the bombers of one group and B-Wings may be the bombers/big guns of another, possibly later recruited, wing of the Alliance.

It might be nice to see the fleet as seen at the end of ESB with a few more ships being joined by smaller fleets from multiple directions to form a larger armada before the briefing.

As it jumps to Hyperspace a few more ships could be added too to make it look like stragglers were turning up from all over the place.

You could even have a few ships jump into the battle after it had already started (possibly a few ancient grubby Separatists ships from the PT redeeming the Neimoideans to some extent).

I like all the thoughts of this quote, but I especially like the bolded portion. That could be fun.