How old were Sting and Rogue relative to the other Dragon Slayers at this key point in the timeline?
Sting and Rogue are the same age as Wendy, in the manga the three are currently about "20" in the latest chapter (I am excluding the +400 years). If my memory serves right, Natsu &amp; Gajeel are both "27". 

Now the problem is working out the age different in the past, this isn't all to hard. Wendy was "12" when she was first introduced (x784). The Dragon Slayers arrived in (x777), so it's 12-7=5. Wendy was "5" years of age. Now Sting and Rogue are the same age as her, so those three were "5" years old. Now assuming I'm correct about Natsu's &amp; Gajeel's age, they would've been 12 at the time. 

Now I'm not sure if this answers the question, but I hope it does. Just keep in mind that Fairy Tail has a habit of making 12 year old kids look even younger than what they are. Easy way to tell is watch the Fairy Tail OVA 8.