Windy and Leknaat are able to remain ageless as they hold half a True Rune each, as opposed to a 'component' rune each. The best analogy I've ever seen to explain was coined by Vextor I believe;
Think of a True Rune as a cup of coffee. In order to make your coffee (Rune of Beginning) you need hot water (Bright Shield) and coffee beans (Black Sword). When combined, you get your coffee. However, you can't separate a cup of coffee back into it's components (as in, you can't turn coffee back into beans and water), but you can half it, in which case you have two half-cups of coffee (Front and Back Gate Runes as held by Windy and Leknaat).
So if your cup of coffee grants you agelessness, it stands to reason that the holders of the half cups will each benefit from this. But your beans and water don't grant it until combined.
Sorry if this explanation was a bit ham fisted, I'm all jacked up with head cold. Vextor explained it much more eloquently but I can't remember where to find his post.