Well, if you want a not-so-serious answer, this question regarding Max's age did come up during the airing of Macross 7. When asked why Max was able to retain his youth after all these years, Kawamori's reply was basically "well, he is a genius..."
As for extending human lifespan via Vajra blood, I don't recall that was ever a thing. Grace & co basically kept a copy of their conscious in Super-Dimension Space and send it to the next host whenever they saw fit. This was based on existing technology within the Macross-verse.
- Tak