A good supporter must function almost as an integral part of your
body if it is going to be comfortable and effective. When choosing
a jockstrap, keep in mind that the sizing does not relate to the
cup size - only the waistband size. Ideally, jockstraps should come
in sizes like women's bras - with a band size and a cup size. Unfortunately,
they don't. However, it's valuable to note that as the sizing goes
up, so does the pouch size. If you think about it, this makes no
sense given the fact that there is no correlation between a man's
waist size and his package size. So, if you try on a jockstrap and
it feels a little snug, you can go up a band size to get a larger
pouch. However, you may have to have the band size altered down.
   

Leg straps should be tight enough to prevent rolling or twisting.
The waistband should fit the contours of your body for natural support.
Generally, a waistband on a supporter will be wider than the average
waistband on briefs, offering greater support. Since the jockstrap
has little to keep it in place, a sturdy wide waistband helps.

  At what age should you start wearing a jockstrap? A boy involved in any sport
that has the potential to cause injury to the genitals should be protected
 - regardless of age. Start them young, and the more used to them they will be.
They will continue wearing them throughout their lives.