Mana is your life force.

People in this world don't age naturally (or, at least, that's not the principle process). What actually happens is that as time goes by, their youth/vitality/health gets leeched away by their spirit-self, bolstering their mana.

Mana is a symbiotic spiritual parasite that slowly drains you of life while bolstering your magical ability.

If you want, you can complete the circle with a full lifecycle for mana. Upon the death of a powerful mage there's an explosion of mana (doesn't have to be an actual explosion, could look like a burst of light or invisible spiritual energy, or even a conscious entity) which then goes on to infect others around them. That opens up a whole boatload of neat narrative hooks. Perhaps you have some people who want to be mages so wind up hunting mages down to steal their power. Perhaps another set of people are so fearful of an accelerated death that they treat it like a disease. Perhaps you could even wind up with conflict within a mage itself between their physical and spiritual selves vying for control (either consciously or sub-consciously). Perhaps up to a certain level of power your body can heal as fast as its vitality is drained so you're faced with a choice: live long in mediocrity or go out in a blaze of magical glory.

So much potential!

Mana is your life force.

People in this world don't age naturally (or, at least, that's not the principle process). What actually happens is that as time goes by, their youth/vitality/health gets leeched away by their spirit-self, bolstering their mana.

Mana is a symbiotic spiritual parasite that slowly drains you of life while bolstering your magical ability.

If you want, you can complete the circle with a full lifecycle for mana. Upon the death of a powerful mage there's an explosion of mana (doesn't have to be an actual explosion, could look like a burst of light or invisible spiritual energy, or even a conscious entity) which then goes on to infect others around them. That opens up a whole boatload of neat narrative hooks. Perhaps you have some people who want to be mages so wind up hunting mages down to steal their power. Perhaps another set of people are so fearful of an accelerated death that they treat it like a disease. Perhaps you could even wind up with conflict within a mage itself between their physical and spiritual selves vying for control (either consciously or sub-consciously). Perhaps up to a certain level of power your body can heal as fast as its vitality is drained so you're faced with a choice: live long in mediocrity or go out in a blaze of magical glory.

So much potential!

Additional thoughts/consequences from the comments:

• Engineering short-lived uber-mages as a last-ditch defence by getting a number of living mages to sacrifice themselves in the presence of one other person.
• Death curses: funnelling all your remaining life-force into a curse at the moment of your death.
• Are mages who intentionally purge themselves of mana immortal? Could be a neat ideological difference between groups
• Human sacrifice to power colossal magical works
• What happens if a mage manages to bind their spirit-self to an object/suit of armour on their death? Do they become immortal/undead, but with no way to replenish mana other than killing for it?
• What happens to mages during warfare/seiges? Does the death of your mages risk gifting mages to the enemy? If you kill a mage in a confined area (seige), could that create a dozen lesser mages who then start building power as their life leeches away?

Mana is your life force.

People in this world don't age naturally (or, at least, that's not the principle process). What actually happens is that as time goes by, their youth/vitality/health gets leeched away by their spirit-self, bolstering their mana.

Mana is a symbiotic spiritual parasite that slowly drains you of life while bolstering your magical ability.

If you want, you can complete the circle with a full lifecycle for mana. Upon the death of a powerful mage there's an explosion of mana (doesn't have to be an actual explosion, could look like a burst of light of invisible spiritual energy, or even a conscious entity) which then goes on to infect others around them. That opens up a whole boatload of neat narrative hooks. Perhaps you have some people who want to be mages so wind up hunting mages down to steal their power. Perhaps another set of people are so fearful of an accelerated death that they treat it like a disease. Perhaps you could even wind up with conflict within a mage itself between their physical and spiritual selves vying for control (either consciously or sub-consciously). Perhaps up to a certain level of power your body can heal as fast as its vitality is drained so you're faced with a choice: live long in mediocrity or go out in a blaze of magical glory.

So much potential!

Mana is your life force.

People in this world don't age naturally (or, at least, that's not the principle process). What actually happens is that as time goes by, their youth/vitality/health gets leeched away by their spirit-self, bolstering their mana.

Mana is a symbiotic spiritual parasite that slowly drains you of life while bolstering your magical ability.

If you want, you can complete the circle with a full lifecycle for mana. Upon the death of a powerful mage there's an explosion of mana (doesn't have to be an actual explosion, could look like a burst of light or invisible spiritual energy, or even a conscious entity) which then goes on to infect others around them. That opens up a whole boatload of neat narrative hooks. Perhaps you have some people who want to be mages so wind up hunting mages down to steal their power. Perhaps another set of people are so fearful of an accelerated death that they treat it like a disease. Perhaps you could even wind up with conflict within a mage itself between their physical and spiritual selves vying for control (either consciously or sub-consciously). Perhaps up to a certain level of power your body can heal as fast as its vitality is drained so you're faced with a choice: live long in mediocrity or go out in a blaze of magical glory.

So much potential!