Discussion Groups / English Usage / January 2007

(After originally submitting this question, I could not find it when I
tried to access this on the alt.usage.english site.  Apologies offered
for a resubmission.)

Can you cite a reference resolving the precise meaning of _th year?
That is, I think I found in Richard Henry Dana's "Two Years Before the
Mast" a phrase that went something like this, "The Pilgrim set sail, I,
in my 19th year, . . . " or some such (I no longer have a copy).

1.  Was Dana 18 years old or 19?  I really don't care how old he was.
I want to be able to infer age reliably from a statement such as the
above.
2.  Most importantly, is there a reference citing common usage?  I
didn't find anything in Fowler or the Merriam Webster dictionares of
English usage.

Failing all this, could you direct me to someone who, or some site
that, or some reference which might address this question?