。
She started taking piano lessons at age six.
Literal
She [topic-は] six-years-old [at-で] piano [possessive-の] practice [object-を] started.
で marks the age at which the action began — a common use of the circumstantial で. けいこ (稽古, 'practice, lesson, training') is written in hiragana here rather than kanji, which is standard in casual writing. ピアノのけいこ is a natural way to say 'piano lessons.'