Social media platforms regularly track, aggregate, and monetize adolescents' data, yet provide them with little visibility or agency over how algorithms construct their digital identities and make inferences about them. We introduce Algorithmic Mirror, an interactive visualization tool that transforms opaque profiling practices into explorable landscapes of personal data. It uniquely leverages adolescents' real digital footprints across YouTube, TikTok, and Netflix, to provide situated, personalized insights into datafication over time. In our study with 27 participants (ages 12--16), we show how engaging with their own data enabled adolescents to uncover the scale and persistence of data collection, recognize cross-platform profiling, and critically reflect algorithmic categorizations of their interests. These findings highlight how identity is a powerful motivator for adolescents' desire for greater digital agency, underscoring the need for platforms and policymakers to move toward structural reforms that guarantee children better transparency and the agency to influence their online experiences.

翻译：社交媒体平台定期追踪、聚合并货币化青少年的数据，却极少向他们展示算法如何构建其数字身份并作出推断的过程，也未赋予他们相应的自主权。我们提出"算法镜像"这一交互式可视化工具，将不透明的画像实践转化为可探索的个人数据景观。该工具独特地利用青少年在YouTube、TikTok和Netflix的真实数字足迹，提供随时间推移的数据化过程的具身化、个性化洞察。通过对27名参与者（12-16岁）的研究，我们发现：通过接触自身数据，青少年能够揭示数据收集的规模与持续性，识别跨平台画像行为，并对算法对其兴趣的分类进行批判性反思。这些发现表明，身份认同是激发青少年追求数字自主权的强大动力，强调平台与政策制定者亟需推进结构性改革，以保障儿童获得更好的透明度及影响其在线体验的自主权。