Mature white pines intercept almost all the sunlight that shines on th

I think what you explain is precisely what option A says.
Option A states that, the ages of trees do not differ much more than the length of time required to reach maturity (5 years, for instance). Consider what the argument says,
1. When white pines mature, little light reaches forest floor. (Premise 1)
2. When there is no light in the forest floor, white pines cannot regenerate. (Premise 2)
3. Conclusion --> Dense forest with nothing but mature white pines. We have to find a reason to support this conclusion.
Imagine a forest with nothing but mature white pines. If the age difference is greater than 5 years, then the pines wouldn't have grown. Tree 1 and Tree 2, lets say have the age difference of 4 years. Tree 1 starts blocking sunlight to the forest floor when it is 5 years old, thus effectively blocking the regeneration of everything in its own shade. But, Tree 2 is already 1 year old. i.e, this Tree 2 is already regenerated, so it receives some form of sunlight and continues to grow.

Hope this helps.

From PowerScore's website:

"Must be True. The correct answer choice is (A)

The stimulus consists of premises, but drives toward a fill-in-the-blank conclusion. You must infer the main point of the argument.

The premises establish that mature white pines obstruct sunlight well enough that such pines cannot regenerate in their own shade, and you need to decide what is likely in a stand of white pines in a dense forest.

You are supposed to pick up on the theme of sunlight-obstruction, and realize that as a stand of white pines becomes thicker, it becomes less likely that new white pines will grow. That would suggest that a dense stand of white pines consists of trees of fairly similar ages.

Answer choice (A): This is the correct answer choice. At the very least, within the stand the pines should not differ drastically in age. According to the stimulus, younger white pines would not survive if they had to grow under the cover of older white pines, so you would expect that a close grouping of white pines consists of trees of fairly similar ages.

Answer choice (B): The area would not have to be cleared of all trees, just enough trees that the white pine saplings could get enough light to grow. White pines cannot regenerate in their own shade, but might be able to regenerate in a lesser amount of shade.

Answer choice (C): The stimulus suggests that older white pines block the sunlight to seedlings, but there is no reason to assume that older white pines would deprive each other of sunlight enough that some of the older trees would die.

Answer choice (D): Since it is quite possible that the obstruction of sunlight makes it impossible for any seedling to succeed in a stand of white pines, it does not follow that other species of trees would colonize and replace the stand of white pines. This choice is unsupported, somewhat contrary, and incorrect.

Answer choice (E): The stimulus does not establish that white pines grow at a fairly invariable rate. Furthermore, the stimulus tends to suggest that the trees are all of very similar ages if they are to survive together, which tends not to support the idea that the height differences should be attributed only to age."

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GMATNinja - can you explain in simple terms why option A is correct?

Mature white pines intercept almost all the sunlight that shines on them. They leave a deep litter that dries readily, and they grow to prodigious height so that, even when there are large gaps in a stand of such trees, little light reaches the forest floor. For this reason white pines cannot regenerate in their own shade. Thus, when in a dense forest a stand of trees consists of nothing but mature white pines, it is a fair bet that______

Which one of the following most logically concludes the argument?

(A) the ages of the trees in the stand do not differ from each other by much more than the length of time it takes a white pine to grow to maturity - CORRECT. POE helps. This one made a conclusion about ages based on time that pines take to grow to maturity, thus leaving no doubts about the conclusion.

(B) the land on which the stand is now growing had been cleared of all trees at the time when the first of the white pines started growing - WRONG. Not necessarily true. May or may not be true.

(C) competition among the trees in the stand for sunlight will soon result in some trees’ dying and the stand thus becoming thinner - WRONG. We are sure about new pines not able to grow in the stand but not about whether the existing ones would die or not.

(D) other species of trees will soon begin to colonize the stand, eventually replacing all of the white pines - WRONG. Irrelevant.

(E) any differences in the heights of the trees in the stand are attributable solely to differences in the ages of the trees - WRONG. Age can be different for two different pines. Looks almost similar to A but it loses for solely relying on age factor.

Answer A.