XS hip protectors are designed for a waist of about 60–70 cm and hips around 85–90 cm, so start by measuring your teen to see if they fall in that range. It's tempting to size up for growing room, but resist that urge—hip protectors only work if the pads sit directly over the greater trochanter, which is the bony prominence on the upper thigh where hip fractures most often occur.
If the protector is too large, the pads will shift off that critical spot during a fall, leaving the most vulnerable area completely unprotected. A snug fit keeps everything locked in place when it matters most.
Instead of sizing up, remember that most quality hip protectors use stretchy compression shorts that already have some give. When your teen outgrows the XS, it's worth replacing them—proper protection beats squeezing extra seasons out of the wrong size.