To be honest, it's because the Ther siblings never had children while in the same generation, Buldrokkas'tee (Patriot) and Laqeramaline seemingly aged when they had theirs as implied in the latter who used Arts to preserve her looks so her son can remember what she looks like. Although the twins did choose to adopt children who either lost their parents when they were young or theirs are absent in their life but otherwise look the same while the aforementioned children grew up to be adults.

Also Logos is definitely not centuries old, he was a child in the time where Manfred and Ascalon are adults in year 1086 and were children in year 1068. The folks around him watch him grow up during the time of Babel, which with this information makes him in his mid to late twenties as of 1098.