I dont think there's a defined age limit
At CURTAINS, I saw many a middle-aged (about to be eldery) women waiting to get signatures (mostly waiting for "Pierce David Hyde" as they called him).
I mean, in my opinion, I think when I'm older and bring my family to shows, I won't be so keen on stage-dooring but letting like my kids and stuff stage-door.
But for right now, when I'm young, I see no problem stage-dooring.
When I'm in my 30's and 40's though, I won't be an autograph hound though :P
I think it depends on personal preference though. That'd just be my preference.
My 2007/2008 Season:
Grey Gardens (7/5)
110 in the Shade (7/6)
Mary Poppins (7/7)
Xanadu (7/7)
Deuce (7/8)
Spamalot (7/8)
Jersey Boys (8/25)
The Year of Magical Thinking (8/25)
Mauritius (11/2)
Young Frankenstein (11/3)
Rock 'N' Roll (11/3)
Pygmalion (11/4)
Mauritius (11/10)
Mauritius (11/21) Mauritius (11/21)
Sunday in the Park with George (3/6)
South Pacific (3/7)
Gypsy (3/8)
Cat on a Hot Tin Roof (3/9)