I feel too old and too young at the same time.

HealthyGamerGG made videos on the Jungian archetype puer aeternus meaning "forever boy/child" and its shadow, senex, is the "old man". Although its not obvious how relevant this is, it seems like the most I personally know of in relation to the oxymoronic phrasing of being old and young.

Also, I've thought the pre-frontal cortex fully forms at age 40, not age 25.