Yea, I give this so much thought it isn't even funny. One of the things I hate, hate, hate about the comics is the sliding time scale.
"Hi, I'm Hank Pym. I'm 32."
"Huh? Weren't you a scientist way back in the 60's"
"Shhh. I'm Hank Pym. I'm 32."
It would be so damn disappointing if the MCU adopted a similar mechanism. That said, let's go over the current set of Avengers one by one. I believe each character needs to be handled uniquely based on the situation and the contract status of the actor that’s portraying them. Again, these are just my thoughts on how I would handle the issue if I were Feige.
Captain America
I believe Cap said that because of the SSS, his body wouldn't even allow him to get drunk. Getting old is basically your body slowly dying, and it wouldn’t be unreasonable for Cap to be aging at a much slower rate than a regular person. Evans has obviously hinted that he might not continue in the role after his contract expires after Avengers 3. So, instead of immediately recasting, I would have Bucky carry the Captain America mantle for a few films, and then bring back a new, young actor as Steve Rogers.
Hulk
This can be written in pretty easily. The Other Guy would probably frown upon Bruce Banner actually dying of old age. Ruffalo has quite a few movies left on his contract… so I think we’re good for another 15 years or so, when Ruffalo actually starts to look too old to play the part. After his contract plays out, I’m thinking they could re-cast Ruffalo with a late-30’s/early-40’s actor while saying something to the effect of, “the green ******* won't even let me age in peace.”
Thor
Pretty much immune to this. Both from the perspective of the character (doesn’t really age relative to us humans) and the actor (Hemsworth’s pretty young and can play the character for as long as he chooses). Recasting Hemsworth, who’s 30 right now, with another actor somewhere down the line shouldn’t be a problem at any time.
Black Widow
Johansson’s the youngest (pre-Age Of Ultron) Avenger, so shouldn’t be a problem with either route they choose. Namely, (a) continue with Scarlett after her contract’s up assuming they reach a contractual agreement, (b) re-cast or (c) retire the character.
Hawkeye
Considering the rate at which Hawkeye’s showing up in the MCU, I think Renner will be under contract for
quite some time… assuming his is a 6 movie contract. He’s 43 now, and by the time he’s done he could really be showing his age. Barton could train a young Kate Bishop, who can take up the Hawkeye mantle down the line.
Iron Man
Now, this is the real issue, isn’t it? If RDJ is open to it, I’d have him continue as Tony Stark for as long as he wants. Tony Stark doesn't need to be a physically demanding character per se, and an older Stark lends a certain gravitas to the character. The 1.215 billion dollar question, of course, is: what if RDJ doesn’t want to continue playing Stark for now? Now, here’s where my idea would differ from the popular consensus of just recasting RDJ and moving forward with Iron Man 4.
I would have Stark retire after his harrowing encounter with Thanos, and continue the series with a new character as Iron Man. The Harley Keener character will be “all growed up” when it’s time for Iron Man 4 (post-Avengers 3), and we could have Stark groom this kid to replace him... for a while. This leaves the door open to RDJ returning as Tony Stark sometime way down the line. Could you even imagine the hype and fury that would surround a Dark Knight Returns-esque Iron Man story that would involve an older Tony Stark played by an older RDJ?
Not that I expect any of this stuff to happen; I'm just speculating away. In any case, here’s hoping they signed Aaron Taylor-Johnson and Elizabeth Olsen to contracts that run 25 movies each.