31 years old, and I have bikes made from 69 to 99. I first rode in 1984, and started paying proper attention to bikes in 85~86.
If I could find a pre-65 bike that was two stroke, had a LHS gear lever, was remotely competitive, and I could afford, then I'd be all over it. But seeing as I don't think such a beast exists, I guess pre-70 is as far back as my participation goes.
If I was really pushed to cut back my fleet, the bikes that would stay are 74, 77, 80, 87, and 94 models.
Edit (for my own clarity, and because all the other kids are doing it):
69 x3
71
72
73
74 x3
76 x3
77 x3.
78 x2.
79
80 x2
82 x2
83
84 x2
87
88
89
94
99