According to rodentpro, weanling mice are 21-25 days old. And "large adults" (is there no such thing as a small or medium adult mouse?) are 30-40 days old and sexually mature.
I separate them when they no longer have that 'baby' look. They look like adults only slightly smaller. Usually about 4 weeks. I HAVE separated them too small before and they ended up starving because they didn't know how to feed themselves yet.
Jay, I would assume in this mice are likely much like rats....the babies will start trying mom's food and her water bottle but also will still actively nurse off her. It doesn't hurt to let them have a bit of time getting their nutrition in both these ways so they are nice chubby weanlings when you do pull them.
Just make sure they are off momma and seperated by gender by 5 weeks of age.