The most common and most likely answer to this crossword clue is the 8 letter word UNDERAGE
I've seen this crossword clue in The Times
U
N
D
E
R
A
G
E
More clues leading to the result UNDERAGE
- Anger after nude running showing a lack of necessary maturity
- University angered cooks lacking the required seniority
- Excessive anger, leaving university too young
- Not yet old enough performing nude at lively party
- Not old enough to destroy deer with a gun
- Too young for nude swimming craze
- Like minor German and European furore
- Young Parisian, one of fashion
- Dean has urge to stray, sex so illicit?
- Legally only of minor importance
Buddy explains!

The straight part
We know that "too young" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case too young = underage.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.
What does UNDERAGE mean?
UNDERAGE adjective- Below the legal age for some activity, such as drinking or having sex.
- Not yet a legal adult; still a minor.
- A deficit or shortfall in funds, inventory, or capacity.