How to look your age?

I know this might sound ridiculous but how can you look your age?

I’m 20 yrs old but to everyone I don’t look like 20. Instead I look 15.

And ppl keep telling me that’s a good thing, and I guess it is but being always told that you don’t look your age ain’t the best feeling tbh.

And worst when I hang out with my younger sister, who’s 15, but yet she looks much older than me.

Not kidding, everyone, literally, EVERYONE always say when looking at me while with her, “You must be the youngest!”

And literally when I say no, they actually put this face:

They don’t believe me!!!! And I don’t blame them, my height doesn’t even help.

So yea, anyone has any idea how I can look my age? Honestly I will do anything!