Her hairstyle makes her look younger than she really is.
Literal
She [topic-は] hairstyle [because of-で] actual age [than-よりも] young looks.
髪型で uses で in its causal/instrumental sense — 'thanks to / because of (her hairstyle).' ~よりも is comparative: より ('than') + も for emphasis. 若く見える ('look young') is the i-adjective adverbial 若く modifying 見える — a productive pattern for appearance descriptions ('looks ADJ').