Make sure they are comfortable because they'll have to wear them while you are sailing and they aren't distracted by swimming or other fun activities.

If you live in a place where it's hard to find a place to try them on in person, I recommend going through the hassle of ordering several, trying them all on at home and then returning the ones you don't like.

We've even found that life jackets that fit our oldest really well, don't do well as hand me downs due to different body shapes. (Not fat/thin body shapes, more like chest size and torso length.)