In the book Frankenstein how old is Robert Walton?

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.