Morgan is 48 he's too old. Downey is 49 and he's heading out because he's too old. They need someone to last another decade's worth of movies.
I don't think it's so much the age - both could easily pass for ten years younger. It's more a matter of waning interest / rocketing slaries.
Morgan (or whoever) would probably jump at the chance, be cheper, more motivated and stick with the part for a good few years.
I'd love RDjr. to do it for the next ten years, but it's not going to happen. Morgan would still be my ideal recast. Besides, Tony's power is in his brain and his super suit. Who says he couldn't carry on into his 50's and further ?