I don’t have any statistics, but one of the voice faculty in our music department, John Stewart, sang as a boy and later had an operatic career, singing at the Metropolitan and all over Europe. He is a tenor.
I’d have to go research this, but I can make a guess about why former boy sopranos might have difficulties singing as an adult. During adolescence the vocal cords – well, the whole vocal mechanism – can undergo a growth spurt, just like the rest of the body. The vocal cords become longer: longest for basses and baritones (about 7/8 inch), shorter for tenors. Children’s vocal cords are about 3/8 inch. The vocal tract also lengthens.
Good singing depends on a balance between phonation (the sound made by the vocal cords) and resonance, plus breath flow and all sorts of other things. If the size and shape of the vocal cords and vocal tract change rapidly, the young man may not learn to adapt and adjust these balances quickly enough to continue singing well. Kind of like tripping over your suddenly huge feet. For some men it can be like having a whole new voice, and they practically have to start their training all over again.
A boy can keep singing in the soprano register while his voice is growing – to a point. But in order to do that he has to increase the tension in the cords more and more to produce a high pitch. I’m not sure if that might be damaging to the voice (except perhaps in extreme cases), however the singer would be so accustomed to the sound, feel, and manner of controlling his soprano voice that attempting to change these ingrained habits would be more difficult. Women do not have as much of a problem because their vocal cords do not grow as much and it is easier to adjust. Tenors would also have less difficulty in that respect (although they a have other technical problems to deal with).
Singers can adjust their resonators (the inside of the throat, mouth, etc.) to produce a more beautiful and better controlled sound. To a great degree this can be learned, but some people just have vocal cords and resonators of a certain shape and size that allows them to work together more easily to create a beautiful sound. When you grow these things change, and a boy lucky enough to have the right combination to sing easily and well might lose that good balance or coupling when he grows. He will need retraining at least. On the other hand a mediocre child’s voice can turn out to be lovely after growing.
Does that help any?