Yes he's actually older than Reed! His memories get jumbled and sometimes completely erased bc as expected dying from betrayal so many times takes quite the mental toll. Sh*t god basically brainwashes him every few cycles so he can "start fresh" ignorant but being the ML, he unfortunately remains lucid at times. This lead to his rebirth as Reed to begin with. Poor Tes ╥﹏╥ the most pitiful ML

Is the-now tesilid the one thats freed from the 100th loop? He said he's experienced not life nor death and the abyss. I just reread it and there was a time where tesilid forgot how to speak and have a darker gaze. Perhaps the disk is what causes him to get to the 17th loop and is now stronger than the tesilid ailette know in the 17th loop?
I think this explain how agnes' friend (forgot her name sorry) was terrified looking at tesilid age, him knowing lots of things that he wasnt supposed to know yet, how he seems like he already know that saving the world wouldnt guarantee death.
But like wowwww if this is the case, tesilid himself would know who reed is and understand him more than anyone bc he's already gone thru that??? Just my theory tho, i didnt read the novel but if this is true im so excited to see the genre change