Michael Rosenbaum
Although he may be viewed as too old to play Flash, there’s almost no other negative to casting Michael Rosenbaum in the role. He looks the part, he’s the most iconic voice of the character, and he’s already got some legitimate experience in the DC universe.
Rosenbaum’s talents aren’t limited to his voice. The guy is an extremely talented actor, despite not being nearly as big of a name as many on this list. The fact he isn’t a huge name is actually a plus. A lot of times it’s harder for audiences to accept a huge name actor in an iconic role. With Rosenbaum, he’s definitely not a no-name, but he doesn’t have a ton of other roles that might create a discrepancy with mainstream audiences.
The one role that may throw off audiences would of course be his role as Lex Luthor in Smallville. That being said, I think his experience on that show is a major plus. That means he at least respects the depths of the DC universe and has a knowledge of it, beyond just what a casual fan has.
In case you don’t pay as much attention to voice actors, Rosenbaum voices Flash for the animated Justice League series from the early 2000s. He really captured the voice and the essence of that character, making for a very believable Flash. There’s no reason why that wouldn’t translate into a live-action portrayal of the character.