当青春变成旧照片
[dāng qīng chūn biàn chéng jiù zhào piàn]
The phrase suggests a sense of nostalgia. 'Youth turns into old photos' means that memories of our youth gradually become distant past events as time passes, and these past moments can only be revisited by looking at old photos. This kind of feeling is usually melancholic with a tint of reminiscence about good old days.