As someone before me implied, each game happens during a different year (but yeah, there are mistakes depending on the website). If you really want to know the age of each girl before playing each game, just check their birth date. Rance's universe has different "eras", for example, Kentou Kanami was born in GI1001: there are 1015 GI years in total after that comes LP0001. Kanami appears for the first time in Rance 1 (which happens in LP0001), she was 15 yo. That's probably why it's on the vndb character page, but it's wrong for Rance VI, which happens in LP0004, so she's 18yo here.
But for example Magic (the princess), which is destined to become one of Rance's "main girls", appears in Rance VI and is 15yo in that game (born in GI1004).
One example of a girl too young for Rance's taste was Kouhime in Sengoku Rance (born in GI1008, Sengoku Rance happens in LP0005 so she was 12). But he didn't wait for long, as I think you can H her in Rance IX or X (LP0006 or 0007, so at best she's 14). If someone who played one of those 2 games could confirm, but considering she's one of the main princesses/wives I assume he Hed her, I could be wrong.