I skimmed the introduction to the internet copy of Orlando, and it said something like Orlando's life is a combination of Vita Sackville West's and her ancestors (it's Vita in the photographs posing as Orlando through the different ages). Vita came from an aristocratic family, one of her ancestors was granted an estate by Elizabeth I (like O in Ch 1) and her grandfather was an ambassador and involved with a Spanish dancer called Pepita of gipsy origin (O in Ch 3).
I think Woolf was satirizing the conventional subject and subject-matter of biographies by making Orlando have "every experience that life has to offer". I agree Ashleigh that she's been very bold in its execution, do you think she seriously meant this book to fall under the genre of biography rather than that of fiction, in spite of its outrageousness?
For me, Woolf is suggesting that time is subjective rather than something which can be controlled objectively by clocks and calendars. Everyone (obviously) has slightly or very different personal experiences. These personal experiences (and our attitudes to these experiences) make it seem to ourselves that we are sometimes older, sometimes younger than how we appear to others.