This rumor scares me because I think there is a severe lack of good young actors right now. There are a ton of good young actresses but not many actors IMO. Nightwing is an awesome character that I want to see be great on screen and the majority of the actors named in this thread aren't good actors in my opinion, no offense to those that like them.
I've also never been a fan of young superhero's, teens in tights is not my thing. I just don't care for it. I'd prefer they cast someone over 25 years old. But that's a personal preference, I just don't like films/tv shows that star young actors very often. Not even when I was a kid. I understand that not everything can be the way I want it to be and that these movies also have to appeal to kids so casting a younger actor might be good for younger audiences but I don't want to see actors that look like or are teenagers running around trying to be superhero's. Just looks corny to me, again, even when I was a little kid I didn't like it. Never liked tiny teen Robin, always liked it better when he was portrayed as in his 20's at least. Hate teen Robin.
My choice would be Ian Somerhalder. Yes, I know he's from one of those horrendous Twilight knock-off TV shows but my ex-girlfriend forced me to watch it a few times and I actually liked his character despite how horrendous that show is (Nina Dobrev is also incredibly hot so that helped distract me from how horrible it is as well). I also liked him in Lost as well. He is older than Cavill but he looks alot younger than he is. I think he would be great and he looks like the Nightwing character. He's by far my first choice.
Alex Pettyfer and Logan Marshall-Green are a couple others I like for the role. Marshall-Green is also older but he can still pass for being a few years younger than Affleck IMO. Pettyfer is one of the few actors under 25 that I like, he'd need to dye his hair but he's a good actor who could pull off the role as well. Tyler Hoechlin is a name I see brought up alot elsewhere. I've never seen him act but he looks the part, he's 26 and doesn't look like a kid, if he's a decent actor I wouldn't be against him. Drew Roy would be ok with me too.
I hate Josh Hutcherson, he's a terrible actor IMO. I don't like Ezra Miller's acting much and he looks too young for me. Don't like Adam Driver or Penn Badgley either. Most of the rest just look like little kids to me, don't want anyone that young. If this is Nightwing, a character that took time to develop over years of being Robin first, then the actor should be in their mid 20's to early 30's IMO.