I might look older, but I'm actually in my first year of college.
I might look younger, but I'm actually in my first year of college.
I might not look my age but I'm actually in my first year of college/high school.
年上に見えるのか年下に見えるのか場合によって言い方が少し変わります。
「こう見えて高校1年生です」
I might look older, but I'm actually in my first year of college.
I might look younger, but I'm actually in my first year of college.
I might not look my age, but I'm actually in my first year of college/high school. 外見と年齢は合わないかもしれませんが、実は私は高校の1年生です。
高校- (米) high school / (英- London) college
こう見えても - I might look like this, but ... / I might seem like I'm ... but ...
イギリス 
