Can go by favourite characters
People who generally stan characters with a bit more depth like Robin, Yonko, Vivi are over 25 or are more mature. Anyone who still talks about east blue or gives a shit about Smoker/Tashigi will also be older.
Anyone who likes Lanji is either a kid or a fanatic at this point. Ivankov lovers are gen z with a 99.999% accuracy or hippies. Luffy, Boa Hancock, Akainu, Mihawk or anyone who has a one dimensional personality so they can find an identity to latch on to.
That's my theory anyway.