Verified answer

Mirai is only 5 but she is already wearing her Ate Sayaka's favorite t-shirt, the one she wore when she was 8 years old.

Mirai was only 5 years old. At the age of 8, she was wearing her Ate (means older sister) Sayaka's favourite t-shirt.

Try changing the "but" for "and she is already".

It would look like this:

Mirai is only 5 and she is already wearing her Ate(means older sister)Sayaka's favorite t-shirt when she was 8 years old.

Mirai is only 5 years old. However, she is already wearing the t-shirt of her Ate Sayaka, who wore it at age 8.