I think Warsman did it since Kevin's life was at stake, and being his mentor, well, you'd figure Warsy would do *everything* he could.
Plus, they were going back to a time period where the only Seigi choujin who was dead was him. None of the other Legends could've gone without risking destroying the Universe.
Warsman was dead at that time? Presumably he was brought back to life again at some point later? So he's probably there to help out the New Gens and save Kevin Mask as far as we can guess then.
Bare in mind that Warsman is a cyborg, i dont think he even gets old.
And as far Neptuneman, he doesnt age because he is a Perfect Chojin? Eh, thats just my theory.
I wondered about whether Warsman could age since he is supposed to be a cyborg and all. But i recall some pictures of his insides from earlier in this tag tournament where he seems to have flesh and muscle encapsulating his internal computer and robotic components. Or are those just synthetic tissue perhaps? He also bleeds as we have seen when Wally's tusk penetrated his body. Could he be a 'techno-organic' being?