Am I experienced enough to share/loan a horse?

I am 14, and have been riding for 3 years, but in those 3 years I have been a sort of project to my instructor, having loads of private lessons and horse care lessons. I am really confident around all the horses in my yard, including the 16.hh ones. The staff always pick me to trial the new horses and I think I am experienced enough to share a horse. But I am worried that the owner of the horse will not trust me becasue I'm young and quite small. And I don't even know myself if I'm experienced enough. But I REALLY want a horse. PLEASE HELP