Besides colour schemes for national flags https://news.ycombinator.com/item?id=24257599 another way to guess ages is when regional flags' symbolism rampantly incorporate flagrantly male charges.

Contrast https://fr.wikipedia.org/wiki/Drapeau_de_la_Californie#/medi...

with https://en.wikipedia.org/wiki/Coat_of_arms_of_Bern#/media/Fi...

or even https://fr.wikipedia.org/wiki/Drapeau_et_armoiries_du_canton...

etc.