I am almost in the mood for my re-read but I don't remember any mention of the quad's age. If channeling causes slowing it is likely those particular kids were slowing long before puberty which implies they are WAY older than they seem. Hence their half sister and brother(Elayne's children) are dead as well as their sister-mother(Elayne). But they barely have progressed in physical age. Add to that that not being a history buff myself the likelihood of an Empire growing to that level in even two regular lifetimes seems unlikely.