How old is Jane at the end?

Hi,
I am writing an essay about Jane Eyre for school. I am writing about religion in Jane Eyre and how Mr. Brocklehurst, St. John, and Helen Burns play into the message of the book on religion. In the part where Helen dies, we learn that 15 years later Jane inscribed resurgam (I will rise again) on her tombstone. My intuition tells me, that at the end of the book 15 years have past from that moment. If that was true, it would really help my argument, so in other words

How old is Jane Eyre at the very end/epilogue?
When Helen dies she is 10, 10+15=25, so is she 25 by the end of the book?

At the first interview with Rochester she is 18, she stays there for a year = 19 and then she runs away, staying in Morton for a year and returning to Rochester = 20. And then the end 10 years later, from that point she has been telling the past . So at the moment she starts the book she is 30.

The story takes up indeed about 15-20 years. I can't remember anymore how old she was when her parents died.

The bit where she's really 30 is only very short. It's the second half of the last chapter where she tells what happened after she got married to Rochester: St John, his two sisters, Rochester's blindness, their first son...