the fact that you're trying to hide your status further amplifies the situation.
take me for example, instead of being an only child, im the last child of 4.
imagine having a brother and 2 sisters as your elders.

people tease me, yea i cannot avoid that. but i try working around it. instead of being a sponge and absorb everything, i at least pull off some punches. like making jokes about someone's nose, or something something.
u see, if you point out something worth making fun of out of the people who makes fun of you, it kinda almost turns the table. and most importantly, they will know that you're not just a sponge.

i try to hide my status, yes i dont doubt that. but when someone knows, i just act like its no big deal.
i never tell, if no one ever asks. if someone knows, and ask me, i just go 'yea, its true. so..?' u know, things like that.

if you have no confidence, even if you're the eldest you still got balls the size of a peanut.