少年我她妈的只是姑娘
[shăo nián wŏ tā mā de zhĭ shì gū niáng]
This username translates to 'In my youth, I am just a girl.' The user might be expressing their youthful identity and perhaps also the contrast between appearance (being young) and inner maturity. There's also a touch of humor in the phrasing which makes it seem colloquial and informal.