Let's also consider that Earth is a lost Jurian colony in the OAV series.
The issue has also sprung up on the manga. (Tenchi Encyclopedia DVD) There
is actually a genetic or maybe a mystical anomaly between Jurian nobility
and common Jurians in terms of lifespans. Jurian nobles live longer than
commoners. Also, it is obvious that Tsunami may have had a hand in Tenchi's
development. There are also hints that Washu may have known a thing or two
about it also. (That's my theory, examine "Hello Baby" a little closer)