There are some good world-building reasons for this. For one, it supports the common D&D trope of "ancient ruins full of the mysteries of a forgotten world." That trope is much less engaging if one of the PCs can say, "Oh, hey, my parents brought me to visit this place when I was a kid." For another, it explains why people are willing to delve into lichdom and vampirism and other forbidden powers to try and escape the advance of age. (Though the existence of the clone spell undercuts this argument a bit, at least as far as liches are concerned.) Finally, it explains why the world is not crammed with ancient mighty heroes.
While I see your point, I don't buy this, personally. Elves live for 800+ years. And, in fact, the common trope for elves is they are reclusive and have very few civilizations that are often hidden or remote.
And I think it would be cool if a PC said, "Hey, I have a vague memory of coming here as a kid...it was 300 years ago! Which, of course, is perfectly legitimate for an elf or a dwarf. I think it would be just as cool as an immortal human who was there 1000 years ago, if it fit the character's back-story.
I do get the argument, "because it doesn't fit the setting" but I see many situations where it fits the setting.
[MENTION=6799753]lowkey13[/MENTION] RE: "So, the question is ... does it matter, or is it just "fluff."
Not exactly. I'm more curious why developers of all editions of D&D find it necessary to make a point of telling players that 'these powers don't actually extend your natural age.'
As someone already pointed out, extended age is a powerful story-telling device. Why make a point to tell players that only high level PCs have access to this device. Why can't a 5th level character have a powerful story? Obviously, it's always up to whoever is running the game but having the designers try to dictate your story tool-box is interesting - especially for something that has no actual mechanical advantage. And why only give that story device to a specific class.
*Note: ...I'm just curious on people's views and the discussion.