Nanaki..is he this or that?

[d£v!l'$ ph0£n!x]

that was way mor complicated than it needed to be

so that means theres human years for every 1 nanaki year. so that means he'll live to be around 300 (human years) so why then in the final screenplay does it say "300 years later" and nanaki appears as a rather healthy mature adult instead of an old man!?