This is what I came across about the difference between 11 and 13:
One girl asked her friend what her age is. Her friend asked her then the question of “do you like to play with dolls or date?” She answered “play with dolls”, “then,” she said, “you are eleven.”
Though with Teena she is a tomboy and has an enhanced intelligence. But still emotionally she wavers between the two as this strip tells us in the title “Pre-Teena”.So it is a bit unfair of Jeri who is 16 to push her like that. Which is why Teena is smart enough to give them to her grandma for safe keeping. She may revisit them sometime later.