Despite having more life energy than others, their bodies become more weak and frail with age. This is a contradiction because the more life enregy you have, the stronger and youthful your body should be.
This is a common misconception among outsiders.
As you spend more time around our people, you'll notice we're generally quite lively to begin with, and our elders are considerably more mobile and independent than non-magical elders. This is because of the levels of mana we cultivate during our lives.
Unfortunately, we're not immortal, and while the body is the instrument of our will, it has limits and age does prevent our elders from enjoying the full physical use of the vigor which naturally accompanies the higher levels of mana they've achieved.
Because of this, they often turn those prodigious energies to scholarship, philosophy, and the deep magics.
Oh, that guy? Yes, he is quite frail. The deluded fellow thought he could extend his life by hoarding his mana. Our elders have as much mana as they do because the years spent using and cultivating their mana has grown their capacity. He didn't do that and his mana atrophied, leaving him in that pitiful state.
Hmm? I think he's around 50. Quite the tragedy, self-inflicted of course, but he had so much potential!
What!? No, the rumors that our bodies have become dependent on mana for basic survival are a pernicious heresy.