Further to what Hanumant has said,
People can grown until their grown plates have fused. An x ray of the hand can help physician's assess what stage of growth a person is at. A person will grow until they have reached an adult bone age.
In thalassemia because of constantly low or fluctuation hemoglobin levels bone age may be delayed, therefore your bone age (and other physiological development) may be delayed. With optimal nutrition and maintaining good hemoglobin levels and low iron levels most thals will reach their genetic growth potential.
It is important to keep iron levels low because high iron levels can accumulate in the pituitary gland and other hormone producing glands resulting in low pubertal hormone levels which can prevent adequate growth and normal development. Antioxidants, by binding with the destructive free iron, will also be protective.
High iron levels can also interfere with other endocrine function resulting in diabeties, thyroid problems etc. So chelation is essential for many many reasons.
I hope this response is helpful,
Sharmin