You can gently ask why and inforce with a non judgemental attitude.
It's always good to ask questions first and wait for answers. Then at least you have a base to work from. As Parali mentioned it could be a number of reasons. A boy, guilt, moms influence, etc.
Also, though you've faded, remember in JW world you're the head of the household. As parent and head you do have major influence. I found when I approach my un baptized publisher daughter in a gentle manner, as a concerned parent first (bringing up nothing re WTS), it usually desolves anti WTS fears (can help w/wifes fears too).
She's too young for you to approach her from this angle....it will also make you look anit WTS to your wife.....". I am looking for any good points I can bring out about the baptismal questions that she will be asked that will make her think."
I would suggest this type of reasoning. Baptizim is a serious commitment like marrige. You can ask if she's feels she's ready for marrige? Wait for answer. Then ask why she feels she's that way (letting her "hear" her own answers). When she's finished you can now add other issues she may have missed. You can also inforce she has plenty time, and "pressure" in doing anything in life, can lead to making choices that lead to problems and stressful situations (giving examples she can understand and she can relate to). You'll also have more of a heartfelt/deeper conversation instead of a dictating one.
If you need to pull out this card, you can mention mistakes now made as an unbaptized youth doesn't hold the same consequence as a baptized one. Right now you as her father help with assisting her with mishaps. Once she gets baptized a whole other dynamic is added where elders get involved and we all know how those issues turn out. You can also bring out how even law enforcement views under 18 as a trial and error stage. After 21 then you're considered an adult with heavier penalties.
Good luck! You sound like an awesome dad