i'm very lucky in that i can reach a 13th with both hands. with both thumbs on middle C i can put my right pinky on top A, and my left pinky on bottom E. although i am left-handed, i find the 13th easier to manage with my right hand. however, i have noticed that many right-handed pianists find that their left hand can stretch slightly further than their right, so possibly it is a trend that the dominant hand has a lesser stretch than the recessive hand. this may be because of a thicker layering of muscle in the dominant hand?
being able to stretch a 13th is very useful indeed. one of my favourite party tricks when with musician friends is to play the opening of rach 2 without spreading any chords.

i am 17 now, and i'm learning brahms 2 among a host of solo pieces by chopin and grieg, some of which i find rather fiddly. grieg has some fantastic un-spread12ths in it though.
to all of you with hands that cannot reach a 10th, do not worry, there is plenty of amazing music out there which is written by people with hands of a similar size.