All of mine used them, because my boobies need a break every now and then lol!
But really, I used them from day 1 and never had any issues with breast feeding.
I took them away between 6 - 8 months old, I found that my kids were ready to give them up around that time, it was a fairly easy transition, and they never ended up sucking their thumb or fingers. My daughter did switch her attachment to a stuffed animal, but she was the only one, and it was a pretty minor attachment.
My mom told me when I was 2 they tried to take mine away, but every morning she'd come in to get me and I'd have one. They finally hid in my room to see what was going on and I woke up, got out of bed and went to a little hiding place where i kept a stash of pacifiers! I just popped one in my mouth and got back in bed
guess that officially means the child is too old!
(And we all have perfect teeth, no braces or work needed at all, and definitely no speech problems unless you count talking way too much!)
Oh, and I couldn't even believe my ears, but my oldest son had a friend/neighbor when he was 8 (the friend was 9). My son kept inviting him over to spend the night, but they kept saying no - finally the mother tells me that her son really wants to but is embarrassed b/c he still uses a pacifier to sleep - at 9!! Didn't even know what to say to that one