I agree that the "milf" tag sometimes applies to pornstars that are actually really young, just like Neutron66 said.
At the same time, I tend to agree that pornstars over let's say 35 y.o. that act like moms on camera should be called "milfs" even if they don't have kids. They COULD'VE had children. And I have friends who've had their first children at 16 or 17, so these girls would have 20 y.o. daughters at age 36-37, so doing mother-daughter-boyfriend scenes isn't that farfetched when the "milf" is 37-38.
Unfortunately, the casting is bad most of the time (e.g. scene featuring 32 y.o. "milf" with a 28 y.o. daughter).