Could thalassemia minor symptoms appear when adult?

Unclear: I'm not sure what symptoms you're attributing to thalassemia minor. Most patients with minor forms of thalassemia are asymptomatic throughout life. If you have developed worsening symptoms of any sort I suggest you have them evaluated by a physician rather than just attributing them to your thalassemia.

No: Thalassemia minor is a genetic finding, and as a result it is present all of one's life. Because it is genetic, it will not get better or worse. It is usually not life-compromising and is treated as a mild anemia. Speak further with your doctor who will know the type thal minor you have.