The most common and most likely answer to this crossword clue is the 16 letter word THIRTY-SOMETHING
I've seen this crossword clue in The Guardian
T
H
I
R
T
Y
-
S
O
M
E
T
H
I
N
G
More clues leading to the result THIRTY-SOMETHING
Buddy explains!
The straight part
We know that "youngish adult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case youngish adult = thirty-something.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.