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