I think he's a good choice. He was amazing in Hurt Locker. I don't think he's too old for the role. Marvel has been thinking out of the box with their casting, and I think he's a good choice. Nothing about the character says he has to be young. And RDJ is 40 right? And he's beloved as Iron Man.
I think they're going to have to cast Cap as much younger than these guys. He was an idealistic young man (early 20s at the oldest) and was only Cap for a few years before being frozen. Having Cap as a young, idealistic man as compared to the older and perhaps a bit cynical Stark would be great. Hawkeye could also be a bit older and maybe an ex-soldier as opposed to thief. Hawkeye's been around and doens't agree with Cap's idealism. Their early relationship was one of tension in the comics.
Regarding Hawkeye's costume. The Ultimate one is absolutely generic. The X-Men movies suffered from generic and uninspired leather costumes. I don't want the Avengers to fall into the same trap. Give them a bit more flair. Certainly, they can come up with something more interesting and bold then a guy wearing clothing that any of us could get at an army surplus store.
Goggles of some form do make sense. Not a fan of the Ultimate mask (looks way too much like WildCAT's Grifter). I would consider him having a longsleeve on his left arm but no sleeve on his right. The asymetry could be interesting and makes sense, since his left arm needs protection from his bowstring. I would avoid guns (I've gotten really tired of gun battles in movies). Having him use a bow is essential, obviously. It would be ideal to have someone new to the team question the fact he uses a bow, and Hawkeye does something amazing to show off. The bow is also underused in modern film. Legolas was rather cool in LOTR but seeing someone use a bow against modern adversaries and kicking their ass could really stand out. Lots of potential there.