That sounds like plenty of teenage boys I knew, to me. They shoot up fast -- before their bodies have had a chance to even lengthen their nerve endings, let alone pack on some more weight (ever wonder why they're so clumsy? ).
But these days, not only do we have bad diets and sedentary lifestyles, but most boys are exposed to a huge amount of estrogen, or chemicals that act like estrogen, through stuff that they eat and eat out of, which often makes them pack on the pounds.
When I was a teen, I got told the same thing, even though I was still within the normal weight range, albeit on the low side. My doctor tested me for EVERYTHING. Even Hepatitis C. Apparently it was just beyond her imagination that an American teen could be a bit on the thin side unless something was wrong.
And ya know, I can't really blame her.