Just look at them. "She's a BABY - you were expecting a giant?" Someone "doesn't believe" her age... tough. It's a fact. I'd tell that person I found it hard believing they were under 70 too, but things happen, right!

My DS is the same size (bit smaller) than his second cousin, who is 14m younger than him. I don't let that get to me: they like playing together and adults aren't the same height so why should children be uniform?