I say, and with God's success: Hijab is required for her upon reaching maturity, and a woman's clothing is restricted to covering the 'awrah and avoiding fitnah (temptation). Therefore, the legal dress is not limited to the jilbab; any loose, non-revealing clothing that does not provoke desire in its details and color is permissible for a woman to wear, especially if the shirt or similar garment is below the knee. And God knows best.