Almost everyone slows down over the decades, and almost everyone attributes the first breathless flight of stairs to exactly that. Most of the time they are right. The problem is that the same sentence — "I am just not as fit as I was" — covers both ordinary deconditioning and the early stage of things that are treatable, and the two are separated by details people rarely think to notice.
The detail that carries the information is the CHANGE
Being out of breath at the top of a hill you have always found hard is not news. Being out of breath at the top of a hill that was easy last year is. Clinicians ask what you could do six months ago that you cannot do now, because the trajectory says far more than the current level does.
Deconditioning is real, and it is faster than most people expect
Fitness falls measurably within weeks of stopping, and it falls faster the older you are. That is the good news buried in the boring answer: the same mechanism that lost it will get much of it back, and the effective dose is smaller than the internet suggests — regular brisk walking, most days, at a pace where talking is possible but singing is not.
The things worth ruling out before settling on "age"
Anaemia, an underactive thyroid, poorly controlled asthma, sleep apnoea and simple deconditioning account for a great deal of new tiredness and breathlessness, and all of them are ordinary to test for. Heart and lung conditions matter more and are also the reason not to self-diagnose from a list.
The signs that are not a wait-and-see
Breathlessness that comes on suddenly, breathlessness with chest pain or pressure, waking at night gasping, needing more pillows than you used to, swelling in the ankles, coughing blood, or a marked drop in what you can do over weeks rather than years — all of those are reasons to be seen rather than to buy something.
What the research does not support
No supplement rebuilds cardiovascular fitness, and none treats a lung or heart condition. Correcting a genuine deficiency helps the person who has it, which is a narrow and testable claim — not the broad promise of energy that gets sold to everyone at once.
A reasonable place to start
Walk most days and keep it slightly uncomfortable. Note honestly what changed and over what period. If the answer is "months, not years", or if any of the signs above apply, that is a conversation for your doctor rather than another search.