I believe old racing frames do not get more flexy as the miles pile up, but they are nearer to the end of their fatigue cycles, and are more likely to crack. They may also be out of alignment. God help you if someone "straigtened" the frame following a bad enough crash.*
Do steel bridges sag or wave more in the wind, as they get older? Does a sports car's frame (if it has one) flex more in hard corners, after ten years of hard driving? A lugged steel bicycle frame does not deform during normal riding, and will remain as stiff as it was when built, until it breaks or is bent.
I am not a mechanical engineer. I am not a framebuilder. I have been riding and wrenching on bikes for "only" twenty years. :) I've broken... maybe eight frames. I am big! I am just some guy.
* During my first-ever uscf race - the Gold Country Criterium in Auburn, CA circa 1988 - I stacked hard. I came to, not sure where I was. I knew it was a bicycle race somewhere in CA. As I limped around the course and saw the finish line banner my memory came back. The frame was an Eddy Merckx Corsa Extra with Faema paint scheme and rounded lettering. I miss it. It had a bent top tube and seat tube and the fork was set back. I recovered, got a new frame, resumed racing. An acquaintance knew I had the frame, played the poor-racer whiny card and so I gave it to him. He said he was going to have it repaired and ride it. I saw it for sale two months later, for half the price of a new frame. He'd done his best to straighten it out, but it still had the same tubes and fork.
If I believed in hell I'd say he's headed there.
Morgan Fletcher
Oakland, CA