Your vet can check your new puppy's knees when you first bring him home. Be sure to ask as many don't do this automatically.
As far a timeline, they can cause LP's by jumping on and off furniture at any time in their life so it's important to have ramps and stairs.
According to the OFA, Yorkies are fourth in ranking for LP's.
http://www.offa.org/patluxstatbreed.html