I have worked “on my feet all day” almost my entire life. I’m 61 now and have only lost 1/4 inch, I cannot tell where it left from, spine or legs or a combination of both.

Yea, my trousers have been shrinking.

The length is still fine.

Yes. I have lost height with age. But even more important is the loss of ability to kick my leg over the level top bar of the frame. I was on a cross country tour with a road bike that I rode around town as a daily rider. I found on tour that I was weaker and my foot would hang up on the top bar when I tried to get on my bike, causing me and the loaded road bicycle to stumble around and fall over on to the asphalt in the highway TRAFFIC LANE, which obviously put me and vehicle drivers in danger. This happened 3 times, and after I made it to a town where there was a interstate bus station I had to abandon my bicycle in a motel, cancel my tour and return home on the bus. I now will not buy a bike with a level top bar frame. I only buy and ride a bicycle with a sloping top bar that I can SAFELY throw my leg over.

lol: that is surely true!

I am 72 y/o. About 35 or 40 years ago I was fitted for a 59 cm Simoncini. At the time I was 6' 0" tall. Now I am 5' 10-1/2", but apparently my legs are still just as long. Back then you fit a bike by getting the largest frame that you could stand over. My stand over leeway is still the same but the handlebars ar a little farther away. My solution was to get a custom made Cinelli Supercorsa. It has a 59 cm seat tube but the top tube is a little shorter than standard geometry.

I was in the pool!!! - George Constanza

Get a shorter stem. Your leg bones don't seem to shrink.

My dad bought a Witcomb from the factory in London in 1991. In 2018 he gave it to me because he could no longer straddle the top tube. I have to admit, the top tube is pretty high off the ground for the frame size. But for context, he was 77 when he passed on the bike to me.

City dock, Annapolis

“I grow old … I grow old …
I shall wear the bottoms of my trousers rolled.”

If the worst thing you have to worry about when you become an "old-timer" (this to the OP) is
the loss of an inch, you'll be doing well.

It looks like I got more than I was asking for in terms of advice! It's good to know these things- finance, taking care of the body, knowing your limits, etc. I enjoy bikes as much as the next person here, which is why I tend to ask these somewhat ridiculous (to some) questions- I want to maximize the time I am able to ride while still young and flexible. For example, osteopenia isn't something I considered when cycling.

Cheers.