This is the 9698th most frequent Japanese word.
若かっ
"若かっ" is the stem of "若かった," meaning "was young."
The word "若かった" is used to describe someone's attributes in the past, specifically their youth.
彼は若かったので、冒険を求めていました。
He was young, so he sought adventure.
"若かった" is utilized to imply a common attribute among a group in a previous time.
あの頃は皆が若かったから、何事にも挑戦したかった。
Back then, everyone was young, so we wanted to challenge everything.
Here, "若かった" refers to a specific period in the past when someone was young.