Alma 53

Helaman says of those he called his sons or as we call the Sons of Helaman - 

"...they were all young men..."  (Alma 53:20)

Explaining that they were - 

"...stripling soldiers..."  (Alma 53:22)

Stripling means - 

"A youth in the state of adolescence, or just passing from boyhood to manhood; a lad."

From that, I would assume they were 14 - 18 years of age.  With this, the miracle of their success becomes more apparent.