Well, you're just fucking brilliant.
The statement wasn't that a lot of 17 year olds sign up. The statement was that there "are lots of soldiers who are 17". That means, right now, there are a lot of 17 year old soldiers in the Army.
That statement remains unsupported...
Forty percent of the US Armed Forces are 25 years old and younger. So, using a simple estimate based on the average age per number of people, the number of 17 year olds is at least 10,000.