。
My daughter is only fifteen.
Literal
Daughter [topic-は] at-last 15 years-old [polite-です].
やっと here means 'only, just barely' (not 'finally'). やっと has two different senses: 'finally' (reached after effort) and 'only just / barely' (reached the minimum). In this context, it's the latter — 'she's barely fifteen.'