Ok... I see how it can be a little different as a working prospect that you're fine returning if need be.
If the pup's hips look good at 15 months.. I'd be pretty confident you're good to go. It's not like they're going to look good at 15 months and then all of a sudden be bad at 24 months. I'll probably do prelims for my puppy at about a year and that will be it. PennHip is also another option that you can do on a younger dog.