I always thought him and Cloud were 14-16 during their time with Shinra. Last I heard, 14-16 year olds around here don't sound like grown men.

It'd be until much later in the game (after the 5 years their were experimented on) that he'd change his voice and maturity perhaps. Especially after witnessing what Sephiroth had done, and gone through the emotional roller-coaster with Angeal.

By the way, it just doesn't sound as fluent as the Japanese voice overs, which is a shame. But I still think the game as a whole will be awesome.