You can find this answer in the second letter of the book. This is a letter that is dated 28th March and is written from Arkangel, a port in Russia.
The letter is written to his sister, Margaret, who is living in England. In part of this letter, he talks about how sad he is that he wasted his younger days. He says he wishes that he would have gotten better educated instead of pretty much just teaching himself.
While he is saying that stuff, he tells us that he is 28 years old.