First: I'm 62, not 17. And this is a book written very well indeed, escpecially for a first try, for young adults.
I think it could be enjoyed by many persons from a clever 7 to 107. Just maybe not all the different parts equally.
So, to avoid spoilers, because even a little could spoil :
Five (5) stars. The Time Travel Mechanics. If it were bread, the flour would come from Anderson and Stross. The yeast from Bester. The seeds on top, the kneading and baking from Mrs Walker. Very tasty.
Five (5) stars. The made up religion. Nasty (but true) bits and pieces from most major religions, and some of their offshots, in a well mixed salad, with good seasoning.
Four (4) stars. The Heroine (17, remember), intelligent, but a bit naïve. Too trusting. Now and again hormone driven (yes, of course). At times emptyheaded (ditto). But sooo much better than Mr Stross's heroine, who is older, educated, etc, but too often a complete ninny. There is teenage romance, very modern, supermoralistic USA style. Try to control your gag reflex. The other characters could take some fleshing out. A side dish.
Four and a half (4.5) stars. Plot. Again Mr Anderson, but also Mr Fritz Leiber (Big Time). Now, Mr Leiber doesn't really take sides. Possibly I am influenced by that, but... I am not sure that this is simply black and white. Well done, Mrs Walker! I see a possibility of more than 2 sides too. At least in the future. Half a star subtracted, because I just can't understand why "The villan" would want to eliminate "x". Doesn't make sense. But otherwise... As complicated as only time travel can get. A worthy main course.
For desert? Not nescessary, there are enough sugary confections consumed by the cast...