What Causes Hair Loss in Women? Especially at an Early Age Like in Their 20s?

There are many reasons why hair loss could occur in a women of any age, including in the 20's. Including genetics and it being natural. But assuming it is a condition factors such as stress, poor diet, hormone changes and blood related issues can also be examined. It maybe worth a blood test to check as this may give a root cause to an underlying hair loss condition. This may well be able to be treated. 

Hey there,
Firstly, hair loss doesn't just affect older people. Since your question stresses on age and since there might be other young people in their 20s and 30s who might find this question and come looking for answers, I thought I might start by pointing that out. 
So yes, it is not typically abnormal, and lots of men and women in their teens, 20s and 30s experience hair loss.
Now as to what causes it - it can be hard to answer that because so many factors can cause hair loss. Change in hormones, stress, improper diet, genetics, medical conditions, illness or even a treatment or cosmetic procedure gone wrong (in some cases)!
The best advice would be to find a good hair loss expert in your city and go for an extensive consultation. This will help the doctor narrow down the causes that affect 'YOU' and what you can do to treat the condition.
Hope this helps.