Is there a method to calculate how much a 20 kg weight weighs, when lifted by me, aged 54, compared to lifted by me, aged 22?

No. The difference is not about your age, but about your body and mind at each point.

There might be a weightlifter who has some standard or even some measure for how hard something feels to lift for each lifter, but there’s no way to map that to age itself.

It seems to me like my experience of the difficulty of physical things varies pretty significantly from time to time based on my physical state, too, even at different times of the week or the day.

You could try to invent one based on your own experience of your own body, if there’s some feeling you can reliably identify. The point where a certain weight feels a certain way to lift. But you’d need a record or accurate memory of what weight felt what way to lift at some point in the past.

@Zaku Thank you for that answer.
I think that my idea of a possible measure method is going in the direction of the “pain scale”.
If one gets explained that scale, one can reasonably securely give their current pain a number (from 1 to 10).
And I might have been on to something, and I think your answer might also point to that, when I stated that it felt like 35 kilos (instead of the 20); that was not ‘made up’, it felt like that to me.
I guess to make such a scale ‘we’ would have to follow one person (of course much more, to get a better result) all their life (from 18 till 100+, and let them lift one amount of pounds/kilos at every stage of their life.
Or am I totally off?

No. None at all.
People are like dogs in this example: I have seen dogs that were 7 years old and their legs were shot. Gone. Old age at 7. I have seen dogs that were 22 and still getting around very well.

If you have a familiar experience of when you experience certain pain sensations when lifting weight in the same way, then yes you could certainly record what weight felt a certain way at what date. Then you could compare that to weights that give you similar pain sensations at different dates.

I was just saying the primary cause is (the details of) your physical condition, which of course varies for different people at different ages.

You gotta hurt yourself dude. But not too much.

Basically you’re measuring variations of effort and instensity – not calculating a static product like horsepower.

The weight weighs 20kg, whether you’re 22 or 54 or 90. So figuring the value of your exercise is subjective, but valid in terms of determining individual stress and related gain in muscle efficiency over time.

As far as an actual formula, there are two common ratings used in athletics and therapy: the Karnoven/Target Heart Rate method, and Borg Rating of Perceived Exertion (RPE).

Google that stuff, or go here to get all the math :
https://www.shapefit.com/cardio/target-heart-rate-zone-vs-perceived-exertion.html.

I’ve already reached my target goals for this question ;)

I like the RPE thingy, from its name alone.
I’ll give them a read, both.
Thank you, @JLoon!

It weighs 20kg, no matter how old you are!

I don’t think age matters, it’s about strength. Most people are stronger at age 22 than 54, but it’s not a given. 54 year olds can certainly build muscle strength barring some specific illness that prevents it.

For myself I would say 15 kg today feels the same as 20kg when I was in my 20’s. For an average man it might not be noticeable, like if I lifted 5 kg or 8 kg. Both are so low in weight that neither take much effort.

X = (mass1) x (Y) x (A) x (B) / Z

X = amount the package weighs at current date
Y = Number of Years since age 22.
A = Amount of arthritis you currently have in your back and arms/hands
B = the ratio of your BMI at current age to age 22.
Z = percentage of exercise you have done every day in between age 22 and current date (1 is an hour per day. Every day missed causes that to lower).

There may be other variables such as how angry you are at the time you snatching and lifting the package today…adrenaline and all that. But assume everything else is the same.

And yes, I just made this up.