I'd say they are younger when cropping. Our Stella was 6 weeks when cropped. 7 weeks for shots, brought her home at 9 weeks.
She is now 15 weeks old, and the posts have been off for a week. One ear is perfect, the other is slightly weaker, but is always standing - just looks a little quivery.
We used Dr. Scholls Molefoam (not Moleskin). Comes in squares, and you cut it to a triangle shape, then roll it to fit the inside of the ear. Glue it in the ear with Skinbond adhesive (hard to find). One band of tape around the base of the ear. The tape usually comes off every 4 days, the foam stays in for 2 weeks. Then the natural oils coming out of the skin causes the glue to start coming off.
The nice part about the foam over other methods (tampons, crack sealer pieces) is that it is flexible - the puppy can roll over on them, and they bend over, and then pop back into shape.
I think that if they didn't take, it's the owner's fault.