On the radio, the boy is described as being 12. Later the woman looking after him says he's 11. And the man posing as his dad says he's 10.
As the movie begins the Chief Gunners Mate's jacket is shown with the rating on his right sleeve and his service hash marks on his left. This is correct for this time period. In subsequent shots they switch places. Currently all navy enlisted rates wear their insignia and their hash marks on their left sleeve. The old "right hand ratings" were in effect until shortly following WWII and represented the original navy ratings.