七年后穿着白裙嫁给少年
[qī nián hòu chuān zhe bái qún jià jĭ shăo nián]
It implies looking back after many years (here specifically 7 years). Wearing a white dress generally stands for weddings or special celebrations, especially in Western cultures. So by saying 'I'll wear a white skirt in seven years and marry a teenage lover' shows her romantic wish of staying true to the love and waiting till finally uniting in matrimony.