My Aunt Denyse is 51 years old.  She recently went on an airplane for
her vacation.  During the trip, she discovered that she has a strong
interest in becoming a stewardess.  My question is:  Is she too old? 
What does she have to do to become a stewardess?  Does it require
training/ school, etc?  Thank you.

It is definitely possible to work as a flight attendant at your aunt's age:

"According to a 1995 AFA survey, 72 percent of flight attendants are
in their 30s and 40s, and 8 percent are more than 51 years old. Delta
Air Lines has 1,793 flight attendants older than 51 and 21 who are
past 64."

http://www.reviewjournal.com/lvrj_home/1998/Dec-14-Mon-1998/business/10192717.html

Note, however, that the older women mentioned above are unlikely to
have been recent hires. Although there are some women in their fifties
working as flight attendants, whether it is possible to be hired as a
trainee at the age of 51 is another matter. In the United States, the
law forbids discrimination on the basis of age, but, as a 56-year-old
woman, I can tell you from experience that such discrimination is
still with us.

Becoming a flight attendant in your fifties and already being a flight
attendant in your fifties are two separate things. Does that mean she
will not get hired? No, the airlines are looking for people who can do
the job and do it with all the stamina, customer care, safety and
security that each flight attendant must do no matter what their age.

Sometimes people decide the airlines are not what they want and,
instead, decide to fly on a corporate jet where they have fewer
passengers and receive better pay. Have your aunt contact the airlines
but also she can look at the Corporate Flight Attendant Community at
http://www.corporateflyer.net to see if something sparks her interest
there. Flying corporate is a very demanding field and she must be up
to the task.