My sister has been learning piano since she was four.
Literal
Younger-sister [topic-は] four-years-old [possessive-の] time [from-から] all-along piano [object-を] is-learning.
四歳の時からずっと ('since the age of four, continuously') chains a specific starting point with ずっと for duration. 習う ('to learn/take lessons from someone') implies formal instruction, unlike 学ぶ which is broader self-study.