He might be angry because your not spending enough time with him. Do your homework in the same room as he is in, if he is in a room without people you need to move his cage. Bedrooms are a terrible place to keep a tiel.

He is in fact quite old for a cockatiel. He could be getting aggressive due to stress and lack of frustration. Try calming him with a soothing voice and whatever anyone says, do not try and add another bird into his cage. Take him to the vet and see what treatment he needs. Hope I helped!

1 year ago he was very sweet and loved everybody but ever since I entered high school, I had less time to spend with him because of an increase of homework. Now he is extremely aggressive and attacks anyone that gets near him. (Also he is approxamtely 7 years of age)