I compared かっこいい's etymology with that of
cool, which has been
attested since 1884. It's not clear from the article when the word gained popularity outside the Black American speech community, but
Google Ngram Viewer suggests its boost happened in the late 70s.
日国's attestation for かっこいい is from the 60s. While 日国 is not tasked to find the earliest one, a quick
full-text search in the NDL Digital Collection turns up a hit for かっこいい from the 50s: 行列はやっぱり馬に乗っていた方がかっこいいなあ (82)*
Even though search noise prevents analysis of hits for 格好いい, there is an interesting attestation of 格好良い:
現代の流行語のように「格好良い」存在 (66)†
As gross as my analysis may be, I'd put the take-off of both words in the same period. If
cool is slang, then so is かっこいい. What complicates the matter is, besides translation gap between
slang and スラング, that WR Dictionaries has a category colloquialism/口語的. What's the difference between
slang and
colloquialism?
*根矢栄一「東照宮の秋祭り」『子どもの四季』秋・冬の巻 石田佐久馬編 東京、福村書店、1954年、80-3ページ。
†長塚慎一「「俺が、俺が」という考え方」『オール大衆』第19巻第15号 (1966年8月)、66ページ。