Re: Downey's last?
Downey is 46 right now. Considering how spry and active he is at this age, I could easily see him not winding down until his mid-50s. So let's assume he's pretty much good to go until about 55.
Avengers, he's 47.
Iron Man 3, he's 48.
That gives about 7 more years for him to appear as Iron Man assuming he'd be too old at about 55. Feige said they're planning sequels approximately 2 to 3 years apart. So that means Avengers 2 and perhaps 2 more Iron Man films after IM3.