Three years ago my gf bought me a Masi Gran Criterium S for my 60th birthday. It is labeled a size 56 but when I measure it C to C it's a 54. That is exactly the size I like. And, the bike in gun metal gray with all the right Dura Ace stuff is beautiful. But, for some reason it never felt perfect when I rode it. Nothing terrible but just not "right." Yet still I loved riding it. Then last season I was riding in the drops and realized that looking down at the front wheel I could see the axle behind the handlebar. Then it dawned on me that over the years my height has gone from 5'10" to 5'9" and that loss is in my upper body. Hey, could my stem be too long now? So, I changed out the 110mm stem for a 100mm and damned if the bike doesn't feel right now. And, it seems that pedaling feels easier. It took me 2 yrs. to figure this out. I feel really dumb. But, happy.

Not so fast with the happiness! You lost 1" in 3 years. I calculate that in ten years as you hit 73 years young, you will be about 5' 6.7" or so. In addition, if every 3 years you need to reduce your stem by 10mm, in ten years you'll need about a 66mm stem.
On the other hand, I tell me wife if you ever can't find me, look in my shoes. I may have shrunk out of sight.

It is not only your height that has changed- the body has aswell. Backs get stiffer-elbow joints are not as Flexible and for some of us- knees give out. I have found that bikes need adapting as I change. Bar height will go up and down as I lose or gain flexibility- Saddle nose up will change a bit and Fore and aft position will change frequently depending on the knees.

Just face it- You are getting old and things are not going to improve. 3 years on the same bike seems a bit long to me.

Investigate N+1 to improve your riding. Probably wouldn't work but might.

As someone born and raised in Hackensack, it doesn't surprise me that you're in NJ.

What everybody else said...but that's not an ucommon experience. i've said for 30 years that i was 6'4", but the other day I measured myself in the doctor's office and I was just a hair under 6'3".
Waist is bigger, though

I just changed out the 10cm stem on my Calfee for a 7cm. Huge difference, much more comfortable! No doubt not as aero, but do I care? No. Still trying to decide if I want to sell this frameset and have Craig Calfee build a smaller one (or maybe custom "Geezer" geometry) for me.

Good to hear you like your MASI Gran Criterium bruce19! I'm still riding my MASI Gran Criterium from '75. Okay, it's a fixed gear bike now (not full Campag Super Record like it was back in the day), but still a great bike!

Rick / OCRR