SailorEris said:
...It's crystal clear they have birthdays without growing old which is, at least, what I want to think.
Even if they grow old though, being 30 does not mean anything for people with lifespan over 1,000 years. Maybe later on they even become immortal, since the time of Sailor Cosmos is in the distant future from the time of Crystal Tokyo, and thus she obviously lives by far more than 1,000 years.