relative age when dealing with long lived races

I mean in the olden days non-humans had legitimate level caps which was an interesting way to overcome the question. I won't says it's the best way by any means, but in the first edition living longer didn't mean you could become the most powerful.

Sometimes we just don't want to deal with it.
I have mostly dealt with it by being vague. Sometimes letting descriptions sort them into a list of ranges.
Instead of exact ages, my players had ranges. Child, young adult, Adult, middle aged adult, old adult, Very old adult (basically 'lets get you to bed grandma) and "Please let me die" for players who have outlived their normal lifespan multiple times over, and are not happy about it. 

For a campaign i was trying to get going, a human king had been rendered immortal, and his Elven arch-druid wife was coming to the end of her lifespan, and he didn't want to outlive her so the players had to find a way to release him. The Elven Arch-druid was in the Old Adult category, and the King was in the "Please let me die" category and i never had to give either of them direct ages. 
The players were 2 humans an Elf, a Dwarf, and the Tiefling Great to the whatever power grand-daughter of the immortal king.
Instead of trying to mat out how the Elf could be " collage freshmen age" like her player, we just went young adult.
The Dwarf was supposed to be 'I just reached the point where i can't be called a child yesterday" as it was his the day after his birthday when it started so also Young adult.
One human was "Father to 3 teenagers" so Adult.
The other human's description was "I am too old for this man" so we went middle aged to old.
The Tiefling was the only one to give her character an age, which was "57"

Still wish that one hadn't fallen apart

Quote from Mog_Dracov >>

Do not forget that life expectancy does not include accidental deaths. Humans have about a 0.6% per year,  accidental death rate or about 6% per decade. 

That’s an order of magnitude higher than modern statistical trends.  The rate was about 54 accidental deaths for every 100,000 people for the US in 2024, or 0.054%.  With a discrete decay of 0.6% per annum, only about half of elves would make it to 100 and less than 2% would still be alive after 700 years {(1-0.006)^700}. Using the modern rate instead, you still get about a quarter of the population dying before they got to 700.  Considering the modern rate is half what it was just 50 years ago and even the 70’s weren’t nearly as hazardous as life in D&D settings, maybe the 0.6% rate isn’t far off unless elves are extremely risk averse.  In any case, you could assume significant attrition after that long.

As for those rare old elves all being archwizards, you have to keep in mind that a PC’s unlimited potential is exceptional.  The vast majority of the population lacks the set of intrinsic characteristics that they need to attain that level of expertise.  If we were suddenly able to double our lifespan, we wouldn’t end up with a population of Nobel-laureate-level theoretical physicists.  More people would have the opportunity to expand their knowledge and improve their skills; but not many people would choose to pursue something like that much less be able to achieve it.  I imagine that elves’ long “childhood” is them giving young elves plenty of time to reach their full potential before they have a permanent role in elven society.  But most adult elves probably spend centuries performing that role without making appreciable improvements to their skills, having already peaked. Since PCs improve quickly, it seems logical that PC elves would often still be in their sub-centenarian decades not being adults by elven standards until after they’re done leveling.  Barring extenuating circumstances, I have trouble wrapping my head around a centuries-old PC that still hasn’t explored their potential.