Theoretically, you are supposed to get your best hair growth between ages 18-24. That's supposed to be peak time. And, you don't get your permanent hair line or growth patterns until your late 20's.
For some people, it's just like the book, but for most I think it varies a lot. I think what Isis said is true to the extent that there are a variety of things which can have dramatic effects on your hair and your health.
The other thing which I have noticed is that a lot of women have some of their best hair growth in their 40's. I remember thinking this when I used to visit HairBoutique.com. The webmistress and a host of other ladies were in their 40's and had fabulous hair growth AND condition.
I think after 25 women seem to deal with a lot of hormonal changes, no matter how subtle. These definitely influence both hair growth and condition. For many, that alone is a very positive change from what occured previously.
I think the time to be especially watchful of your hair in terms of the hormones is when you are getting close to menopause or actually in menopause. Some women say that this is the time when they notice extra shedding and a change in the condition of their hair.
~Jade~