1. Bradley Cooper - too old, plus he's already Rocket Raccoon
2. Chris Pine - borderline too old. He's a year younger now than Downey was when Iron Man came out, and Downey was borderline too old for the role then.
3. Ryan Gosling - same age as Pine, but has aged better. He's on the border line age-wise but is the perfect fit in every other way.
4. Joseph Gordon-Levitt - A year younger than Pine or Gosling, but doesn't fit the role at all. I don't think he can believably play Tony Stark.
5. Colin Farrell - Too old, and a terrible fit for the personality of Stark. Worse even than Gordon-Levitt.
6. John David Washington - never heard of him. He's Denzel's kid and he used to play for the Rams. He's younger than anyone listed so far, but having never seen him act I have no idea how he'd be.
7. Nicholas Hoult - the About a Boy kid, all grown up. He's the perfect age for the role, but he's not manly enough to be Tony Stark. Stark is a throwback to another era, so unless their going to go with more of the modern autistic goofball techie archetype, I can't see Hoult in the role at all.
8. Matt Bomer - another guy I've never heard of. He's too old. Plus, he looks more like Steve Rogers than Tony Stark.
9. Steven Yeun - Maybe he can pull it off. I've never seen him as the suave playboy type, but he's impressed me as an actor in general, so I'd be willing to give him an audition.
10. Alden Ehrenreich - They saved the best for last. He's better than anyone else in the list, and he's young enough to keep the role for the duration of another 30+ movie franchise. If Gosling were younger, I'd go with him, but of these 10 choices, Alden Ehrenreich is the one.