Teeth whitening treatment for teenagers..!!

Hey guys,
My daughter is 14 years old. She is conscious of her appearance, smile and all. For the past few weeks, she has been complaining to me that her teeth look yellowish. And yes, it seems yellow to me also. Is it normal to have yellow teeth? As she is conscious of her appearance, she used to maintain good oral hygiene. So what could be the causes of yellow teeth? She is asking me to take her to any dentist for considering dental whitening treatment in Guelph. Can a 14-year-old get their teeth whitened? Is there any age limit for teeth whitening? I would like to hear from you before making any final decision. It would be a great help to me.