I use these socks for my three-month old, with the tops folded down.  They are very cozy and so far are the only socks I've found that stay on!  I recommend washing them inside out to get rid of the lint so it doesn't end up between their toes.
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Update

At nine months, my baby no longer wears these socks much.  With the new found mobility, these socks, like many others made for babies, simply won't stay on!  They also don't have an anti-slip coating, which means that they are not suitable for walking.  We do use them as a second layer for walks in the stroller on top of baby's regular socks.  They are still very warm and cozy!