And we still don't know exactly how micloned Meltrans age---Milia's the first, and is basically "an experiment". I wouldn't be surprised if she shows up looking exactly the same as she did in M7.
How old do Zentradi live? Britai was very old IIRC. But he's "special" and not your typical Zentradi. Milia's Meltran, for starters, and is presumably "more special than average, but not as special as Britai/Laplamiz".