。
How old will you be next year?
Literal
Next-year you [topic-は] how-many-year-old [into-に] become [question-か].
何歳 ('what age / how old') uses the counter 歳 ('years of age') with the question word 何 ('what'). ~歳になる ('become X years old') is the standard way to express age changes — Japanese says you 'become' an age.