While they live in a modernized time, it never explicitly says that they live in our time and could actually be in the future. Draculaura is 1600 years old and predates Dracula's actual date, not to mention Cleo pre-dating mummification, but only if they were in this year (2010/2011). The diaries also never state a specific year. Ignoring the book series continuity, it would make everything fit and everyone can be the ages that are claimed.