50's could still go either way, but by 60 definatly can't be counted as milf, to be clear 45-60 is mature, but milf can still apply, 60+ is old but potentinally bangable
When she seems too old to be the mother of a teenager who'd use that expression. Well over 60. That's where the "MILF" expression came from, the 1999 movie American Pie where it was used by highschool seniors to refer to another's mom who was exceptionally still fuckable/desirable despite being old enough to have had and raised a teen of her own.