I think the thing with it is, it's going to be so long until she can wear it, would you want something you (or your partner) can wear until you pass it on? Or would it just sit in a drawer/safe for the best part of two decades? If the latter then maybe it makes more sense to buy yourself a watch to celebrate now (and to remind you of her every time you look at it), and then something to mark a significant occasion in her life - 18th, graduation, wedding, etc - when the time comes.
Hey, congrats! JF answered a similar question:
https://www.hodinkee.com/articles/winding-weirdness-watch-wear-wisdom-and-heirloom-watches-part-deux
I think the thing with it is, it's going to be so long until she can wear it, would you want something you (or your partner) can wear until you pass it on? Or would it just sit in a drawer/safe for the best part of two decades? If the latter then maybe it makes more sense to buy yourself a watch to celebrate now (and to remind you of her every time you look at it), and then something to mark a significant occasion in her life - 18th, graduation, wedding, etc - when the time comes.