Teens seems to also mean griefs. If you read it that way, I get the feeling it might mean something like "I've watched her grow from a carefree child to a somewhat more troubled maiden". Even so, I still don't understand the arithmetic bit, ha ha...
Cleocima please ask in Phrases&Sayings, there are people there who will sink their teeth into this with great pleasure.
Just occured to me that 'teen' meaning grief and misery may not necessarily make 'teens' the plural form of that sense of the word.
Cleocima if you don't receive an explanation in Phrases&Sayings but you do find it elsewhere, would you be so good as to post it here? ...'cos I can't stop wondering now...!
Thanks again swedeheart for your concern. I am fully conversant with the second meaning of 'teen'. I have now given this a lot more thought and I believe it could well mean something like: could number five (more on) from ten, making her (Olivia) now 15 going on 16. A flourishing teenager.