I have an older cousin (who I haven't seen in years) - but I remember one time when I was quite young maybe 6 and she was probably 16 or so and someone had commented, "you both look like you could be sisters, you both have that sour puss on your face!" and my cousin snapped back about how it's just her face and that she wasn't sour.
25 years later I learn the term resting bitch face and I've adapted it and embraced it. It's my face - quit telling me to smile (I don't want to, it hurts and you're lame), quit telling me to stop looking so annoyed (I can't, I am annoyed) and quit asking me if something is wrong (you're my something wrong)!
I will also note that I'm constantly told that I have a baby face and am usually mistaken for as a college kid (I'm 30) - there is obviously truth in this statement.