Where (in the US) can I purchase a copy of the program, "Age Seven in
America" which was shown on CBS in the mid '90s and its follow-up,
"Age Fourteen in America?" In the first program Merle Streep
interviewed children who were all age seven; in the second the same
children were interviewed at age fourteen.  This second program was
shown on PBS about a year and a half ago.

From what I can find out, these videos are currently not available for
purchase.  "Age 7 in America" and "14 Up in America" originally aired
on Showtime in, respectively, 1991 and 1998.  It's possible that when
"21 Up" is created and shown in 2005 Showtime will replay the first
two films.  But that requires you to wait around for a few years.

Age 7 in America (1991)
http://us.imdb.com/Title?0281550

14 Up in America (1998)
http://us.imdb.com/Title?0190212

At least one place (the University of Georgia library system) has a
copy of 14 Up available for viewing, but obviously that's not one for
sale:
http://www.libs.uga.edu/peabody/aatv1998.html

I'm not sure if you're aware of it, but the U.S. version of 7 Up was
based on the very popular UK series that started back in the '60s.  
The latter films in the series (all directed by Michael Apted) are
available on video/dvd and are recommended.

The UK version:

42: Forty Two Up (1998)
http://us.imdb.com/Title?0164312
35 Up (1991) 
http://us.imdb.com/Title?0101254
28 Up (1985) 
http://us.imdb.com/Title?0088650
21 Up (1977) (TV) 
http://us.imdb.com/Title?0075610
Seven Plus Seven (1970) (TV)
http://us.imdb.com/Title?0066356
Seven Up (1964) (TV)
http://us.imdb.com/Title?0058578


leep-ga

As a fellow researcher, I would like to confirm leeps findings. I did
however find a copy of one of the tapes - but it is not in the US as
you had wished.

http://www.btinternet.com/~concordvideo/ch45.html

I am unsure if the tape still exists in this capacity, but you can
contact the site owner via this contact page link:

http://www.btinternet.com/~concordvideo/cp5contact.html

I will keep my eyes open for you as I conduct more research -

SgtCory