。
She looks at least sixty.
Literal
She [topic-は] at-least 60-years-old [as-には] appears.
少なくとも ('at least') sets a lower bound on the age estimate. The には combines に (degree/amount) with contrastive は, emphasizing 'at least this much.' 見える (みえる) here means 'to appear, to look like' — a spontaneous perception, not an active choice to look. The sentence estimates appearance-based age.