I realised something. So after watching The Timeless Child, we’ve got to know that the Doctor is essentially immortal. She has unlimited regenerations.
But if y’all remember, in Time of the Doctor ( Eleven’s last episode) he was ageing and dying in Trenzalore cos apparently, he had used up all his 12 regenerations. Therefore, Clara had to beg the Time Lords to grant him more regenerations.
Now, if the Doctor had unlimited regenerations, as she/he was the whole thing behind timelords, how could he age?