
Don't know about your breed but in mine a nice together puppy will more often than not beat a Junior at the stage where they are at the rangy stage having lost their puppy coat and appear to have lost all their bone.
My Jozi is always so OTT that she needs to go around the ring a few times to settle. If the judge does not move the class as a whole I can forget it, as she will fidget and play up for her exam and when I do her triangle, and especially the up and down as she hates it. She loves to go around a big ring best.
When she was a Puppy and Junior I always entered several classes.
At her first Club Championship show as a Junior she was in five classes. The judge forgave her high spirits in Junior which she won, she was better again in each subsequent class, and won them all, and then her first CC, and RBIS.
I like to try the next class up when the dog is nearly out of their age class to test the water against the older competition.
Also in numerically small breeds it can help you get a better idea of the quality of your dog.
A dog I bred came fourth in the well filled puppy class at Crufts one year but won the Junior class of four or so.
Occasionally you might be in a small class and get a second and third and not know if that meant the judge didn't like your dog, or just preferred the other. I have had a second in my first class of two and then won the next one with more exhibits.
Of course I don't go to shows every weekend so the dogs never get a chance to get bored or overtired.