The phrase "to become mobile" means to acquire the ability or capacity to move, especially by developing the power of locomotion or self-propulsion. Full definition
The two lower positions are used when baby becomes mobile and can sit - up. (simplybabyfurniture.com)
It was pure physical expression, made by those whose head didn't carry too many thoughts — at least, not when they woke and became mobile again. (bookbrowse.com)
I don't think I'll start now, as my baby is 7 weeks old, but when he starts becoming mobile... we're on it. (babycenter.com)