Despite how young the heroine, Gwen, seems to act versus how old she actually is (almost seventeen but acts like she's twelve), she is not the biggest issue I had with Ruby Red. No, the biggest issues I have are 1) how boring it is, 2) how nothing seemed to happen and 3) the vague worldbuilding and unexplained... everything, basically. 
 All her life, Gwen's family has prattled on and on about how special her cousin Charlotte is because she has the time travelling gene (which is never explained). So when Gwen is able to travel through time instead, it comes as a shock to everyone. 
 Barely anything happens, and I'm not talking action sequences. I mean nothing really happens that propels the story along at a decent pace. After Gwen's initial time travelling panic, she and Gideon travel twice to meet other time travelers, one who is obviously evil (and really, who didn't see that one coming?). But it's just idle chit-chat with some unexpected yet expected danger near the end just before they go back to the present for safety. More time is spent on describing Gwen's clothing for the period she's going to travel to than anything important. 
 The characters are all flat and hard to connect with. Gideon in particular, because he's really just kind of there. Gwen thinks he's in love with Charlotte but at the end, out of nowhere (view spoiler)[he declares his affection for Gwen. (hide spoiler)] Gideon is just a pretty face and Gwen constantly reminds us of this, and she barely seems interested in him. Combine the fact that this book takes place over the course of a week, tops, and we're given a case of insta-love. 
 The worst is the time travelling aspect. Listen, time travel is a bitch to write, okay? There are so many things you have to consider and it seems like nothing was in Ruby Red. (Spoilers for the ending) (view spoiler)[If Paul and Lucy warned Margaret about travelers from the future coming to take her blood, how was her blood acquired initially for the chronograph? If Gideon is right and Future!Gwen told Lucy/Paul/Margaret that they would be coming for the blood, why didn't they know that Gwen and Gideon would reject their pleas and run? If Saint-Germain knew the chronograph was going to be stolen in the future, why didn't he leave a warning against Paul and Lucy's intentions? (hide spoiler)] THIS is why time travel is such a pain in the ass. Even if Gier is using the Fixed Timeline theory of time travel, so many things don't make sense and makes my brain hurt. Multiverse for the win, basically. 
 I get that this is a trilogy. I'm writing a trilogy and you can't explain and resolve everything in the first book. However, nothing is resolved in this book and all we really find out (view spoiler)[is that Gwen is Lucy and Paul's child, not Grace's, which also makes Gideon and Gwen cousins. Kissing cousins. Paging VC Andrews. (hide spoiler)] 
 As I usually say: idea good, execution poor. We'll see about reading Sapphire Blue, because I wasn't that put off by everything.