The most common and most likely answer to this crossword clue is the 6 letter word NONAGE
I've seen this crossword clue in The Guardian
N
O
N
A
G
E
More clues leading to the result NONAGE
- Minority, say, soon getting sent back
- Minority of lawyers give a 90-yr old 50% off
- Nobody detains a girl below the age of consent
- Silver service’s passed round for youth
- Some began onanism around a period of immaturity
- Early phase shown on a genome sequence
- Youth and Orangeman marching around without sheep
- Minority of lawyers could be 90-year-old — not half
- No worry with energy at a minimum in early period
Buddy explains!

The straight part
We know that "period of immaturity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case period of immaturity = nonage.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 NONAGE mean?
NONAGE noun- The state of being under legal age; minority, the fact of being a minor.
- A payment formerly made to the parish clergy upon the death of a parishioner, consisting of a ninth of the movable goods.