It's hard to tell since they don't specify but assuming I am guessing my guess is right ( I am not the best at guessing people's ages（/TДT)/ ) she is either 13 or 14.
Hopefully the manga will specify later on her age in her new current world because their relationship is cute and adorable and I don't want their being any issues interfering with their relationship.(๑•ㅂ•)و✧
Although if they wait till she gets seemingly older before they deepen their relationship I am TOTALLY fine with that (︶︿︶)و✧as I don't mind stories that take time with the romance developing between two characters.
That's what timeskips are for (●￣(ｴ)￣●)
She died when she was 24, but how old is she now? 8 or 16?