This question comes up here quite frequently.
I've never really understood why. I get the idea that because there's so much pressure on students to perform, or to appear "advanced" in some way that this somehow translates into needing to follow a neatly linear academic trajectory towards a PhD, or that the only way one can make a meaningful scientific contribution is if one finishes a PhD before age 30.
But the reality is that everyone has his or her own unique set of circumstances to deal with. Sometimes you have to take a year off to work. Sometimes you get sick, or pregnant or have to become the primary caregiver in your family. Some people don't even realize they enjoy a subject until they're a little more mature.
Life is all about decisions then.