I think you overthink the whole situation... Yea you do look young but who cares? You shouldnt care too much what others think... If they dont take you seriously than they're asses! Your time is more worth than wasting it for them...
And about the people you're dating on the internet... If you look at the situation objectively like if you get a picture of someone looking too young than they are its a normal reaction to ask if thats an old picture, isn't it?. I dont think they tryin to be mean... I would ask too... And like in 10 -15 years or so all of them will look old but u'll still look young and they're gonna be jealous!

At least thats what I think...
I hope my thoughts helped you in some way.
Have a nice day!