It could well be that this protein is good for you when you’re young but not that good for you when you are older.
For example, young people might encounter more new infection sources, and thus need a faster/stronger responding immune system. This protein might be evolved for giving you that, with a side effect of having too strong an immune system at older age.
Evolution may not yet have found a solution that turns down its production at later age, or it might have evolved it at some time, but found its benefits do not outweigh the cost of maintaining the necessary control mechanism.
It’s far from a given that having more humans live to old age has evolutionary benefits.
For example, young people might encounter more new infection sources, and thus need a faster/stronger responding immune system. This protein might be evolved for giving you that, with a side effect of having too strong an immune system at older age.
Evolution may not yet have found a solution that turns down its production at later age, or it might have evolved it at some time, but found its benefits do not outweigh the cost of maintaining the necessary control mechanism.
It’s far from a given that having more humans live to old age has evolutionary benefits.