My dear , I had the unlimited good fortune of sitting at the lotus feet of Srila Prabhupada hearing him give a lecture on March 2, 1975 in Atllanta, Georgia USA. In this lecture he explained as follows:
"Just like Kṛṣṇa, the Supreme Being. He must be very old because He is the original person. But the Brahma-saṁhitā says, advaita acyuta anādi ananta-rūpam ādyaṁ purāṇa-puruṣa nava-yauvanaṁ ca [Bs. 5.33]. Purāṇa-puruṣa, the oldest person, but you will find Him nava-yauvanaṁ ca, always a fresh youth. That is God."
Here is the verse that he quoted from:
"advaitam acyutam anādim ananta-rūpam
ādyaṁ purāṇa-puruṣaṁ nava-yauvanaṁ ca
vedeṣu durlabham adurlabham ātma-bhaktau
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
I worship Govinda, the primeval Lord, who is inaccessible to the Vedas, but obtainable by pure unalloyed devotion of the soul, who is without a second, who is not subject to decay, is without a beginning, whose form is endless, who is the beginning, and the eternal puruṣa; yet He is a person possessing the beauty of blooming youth."
Materially is not possible to be simultaneously the oldest and youngest of all persons. But spiritually it is quite possible.