Charlize Theron shaves her head for Fury Road

[zykotec]

When she was younger, and most peoples hair get darker as they grow older.

[Chase Bansi]

Ok, so her natural hair is dark brown/black. I don't think so. People with blonde hair can also have dark roots. As a professional hair stylist you should know that hair can change color.

Yes, as a child I was blonde which darkened as I got older. True with many in my family. As hormonal changes happen in the body the production of melanin increases darkening the hair.

In Summer some kind of people with dark hair can turned blonde.

What you are seeing is NEW hair. It is only a few weeks growth (hair grows approx 1/2" per month) and has not had the prolonged exposure to sun (or pool chlorine) for it to become sun bleached. As you have pointed out their hair turns blonde. It is not 'naturally' blonde.
The picture posted by Zykotec proves that even when she was younger she had dark hair and was not blonde, nor did she have even 'summer blonde'.

[Bubba Fat]

Ok, you're right. In Summer my hair tends to get reddisch brown instead of dark brown/black. But yep afete the pic I saw, she's in natural dark brown/black and not blonde haired.