Time is tricky in Boys Over Flowers since it takes place in a floating timeline, which "explains why [the] characters age little [...] over a period of time". While eleven years passed between the series' debut in 1992 and end in 2003, only around two years actually passed in universe. Unlike the manga, the television dramas Meteor Garden (2001), Hana Yori Dango (2005), and Boys Over Flowers (2009) are on a fixed timeline.
All ages listed on manga character pages are from before France Story. Ages have to be sourced and not guessed because of the floating timeline. For example, Amon Kunisawa's age is listed as 21, though he has most likely turned 22 or 23 by this time.
Ages of Season 2 characters are current, since only a few months have past in the series.
Birth years are included on several character pages, however these years are considered only partially canon. All the birth years were published before Yoko Kamio established the floating timeline of the series.
All ages from Hana Yori Dango are sourced from the official website. Some ages from the sequels, Returns (takes place about a year after) and (about five years), are guessed as they are not listed on the websites.