The Magician's Son by Barrington J. Bayley, from White Dwarf #136 (1991).
"There are ways to become immortal. There are Champions of Chaos that
live in the realm of the Chaos Powers, but They are no longer human.
You can become a vampire, one of the undead, but that is like a living
death. No, our friend wanted to enjoy life forever as a normal human,
to stay reasonably young indefinitely. Well, there is a foul way
to do this. Every time the adept begins to age, he must sacrifice
a child and absorb its youth, its life forces, into his body. But
not just any child will do. It must be between six and twelve years of
age, and the same sex as the recipient. Most important. the operation
will not work at all unless the child is the magician's own child."
Mucnchbek's eyes smouldered. That's why Semperphilius was always so
anxious about the boy. It was his own life he was protecting!
