In fantasy setting, in medieval time, you can find an illness, curse. for convenience in this situation, why don’t we phone it “vampire”.
The target who’s suffering from this “vampirism” does perhaps not grow older. Therefore then he/she will be a child forever if he/she is a child. He or she doesn’t perish of later years, but only die when killed, from a major accident, starvation, and comparable non-age causes that are related.
They will have some restricted powers that are special greater strength, sharper sensory faculties, rate. The longer hey live, the higher their powers become (their energy might perhaps not develop in natural energy, but might develop in ability by regularity use/training). Nevertheless, the ability is explainable as mundane, it generally does not reach supernatural levels.