mozart never had a wheel as a baby and has never shown any inclination to run in one since. picasso will marathon for 3-4 hours a night every day, and it would break her heart for me to remove it (sometimes it gets pushed out of the stand; it's a mesh-wire wheel, and she seems at a total loss until i repair it, whence she will jump in and begin to run no matter what time it is).
picasso is rather small for her age, i think, and she is like 6 months old in a 9" wheel. it's getting a little small, but the 12" silent spinner is so gigondous that i'm going for an 11" wodent wheel, just so you can get a feel for how big of a wheel you will eventually need.