How old is your 747-400 fleet, 200 fleet and A320 fleet?
Per the "Aircraft Fleet Information" pamphlet put out by NW dated April 1, 2008:
747-400:
-6 with date of manufacture of 1989
-4 with dom of 1990
-4 with dom of 1999
-2 with dom of 2002
747-200:
-2 with dom 1975
-1 1977
-2 1979
-2 1980, 1 -1981
-2 1984
-4 1987
-1 1988
A320:
Not gonna list all 73 but the majority are 1992-2003. 27 are 1998 and newer.
A319:
All 57 are 2000 to 2003, except for 2-1999.
A330:
All 32 are 2003 to 2006
Detailed enough?