I've had 3 different experiences - DD1 stopped at 3 mths, DD2 at 9 mths, and DD3 at about 14 mths. i don't think there's a rule saying that any age is too old, just what your baby needs.

I started using a single bed sheet instead of a wrap and then bought some stretchy muslin from Spotlight about 2m by 1.5m to wrap the older baby. I wish i could wrap her now as she may sleep better, but it's too late to go back! (she's 19mths).....