You were there to help a young relative /teenager decades ago ...

Maybe the older person, being now much older after 40 years, brings it up as a form of reminiscing, a sort of, "I remember when..." and there's no underlying unpleasantness implied which I wonder is what you're thinking?

Unless it's an elderly family member we know, one of whose recent favourite threats sayings is,
"I always look after those as look after me," with the implied threat of their will money hanging in the air to keep people dancing attendance... 😒

Is this the older relative who talks about money all the time?

If so, I would put it down to the fact that for some reason they are insecure about money, and as you have always had a happy and healthy relationship try to focus on that. Perhaps they want to know that they will be remembered kindly, and are concerned that people will think of them badly after they die, or they are worrying about the afterlife so are reminding themselves as well as others that they did good deeds.

As the older person gets older, probably frailer and realising this, they keep talking about the past help as a way of reminding younger relative that yes, they were there for them when it was needed, and hinting that it’s possibly payback time.

“After all I’ve done for you….”

I had a special guardiànship order for my DGD from the age of 4 and was a kinship carer for her,she is now 25.
I have helped out,covered her mistakes,got her out of scrapes,all par for the job description,still doling out the odd tenner and treats.
Would I rub her nose in it ...definitely not.
I do it because I love her unconditionally.
I do have ' a word' in her ear occasionally about budgeting, but all done with love ,support,and a hug.

bringing it up 40 yrs later on a guilt trip is exactly that, a guilt trip.

doodledog - no it's someone else - and totally unexpected that they should talk about the other younger relatives in that way.
We see what goes on and it's not the way the older relative misrepresents it.
It's not difficult (when someone had the means) to put a fiver in an envelope, but (on the other hand) it's more generous of their time, skills, their energy, loyalty, discretion and grace to do what the lovely younger relatives now regularly do to support them in all sorts of emotional and practical ways. Too many ways to mention.
It shouldn't be like this and is quite sad that they have this sense of tit-for-tat entitlement today.
What was thought to be offered out of love at the time (''don't forget to ask if you need help'') clearly wasn't the case at all - as it's all been stored up in meticulous detail as credit further down the line !! The younger generation are far more straightforward at times.

seems a very strange thing to do.. who on earth remembers spontaneous gifts to loved ones..

When I’ve helped out in this way I’ve always made it a condition that it’s not mentioned again and I never do, although the recipient will occasionally refer to past kindnesses with gratitude.