She's old in years, not in body. Boss Monsters don't age until they have kids. Then the kid drains their yearly progression out of their parents more or less until the parents die, the new Boss Monster is an immortal adult until they too have kids.

And Toriel and her husband Asgore lost their son, he died along with his adopted brother. They became immortal again after only a few years. So she can be like, 3000 years old but still more or less late 30s.