The most common and most likely answer to this crossword clue is the 8 letter word LIFETIME
I've seen this crossword clue in Independent
L
I
F
E
T
I
M
E
More clues leading to the result LIFETIME
- A very long period, hopefully encapsulated in two sentences
- The seven ages of man in two sentences
- Years when fruit contains within it iron
- Type of guarantee you can’t pass on
- Cradle to grave fee limit reviewed
- Biographer’s topic, sentence after sentence?
- Around 82 years? Sadly I feel it around end of swim
- Type of subscription bundling together two magazines
Buddy explains!

The straight part
We know that "The seven ages of man" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the seven ages of man = lifetime.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 LIFETIME mean?
LIFETIME noun- The duration of the life of someone or something.
- A long period of time.