I believe he was 10, but I think that depends on whether you read the Roy Thomas Captain Marvel series or the John Ostrander Power of Shazam series. But if I recall correctly in power of Shazam, the original graphic novel, they retconned Thomas’ story and said Billy was 10, then in the ongoing he was like 14 or 15 because they also acknowledged his time in the dc universe like with Justice League International.