“By the time”: what time case to choose?

Question

I have the following sentence that I doubt about what time to choose:

By the time they are in their late teens, their ability to learn ____.

What should I use?

1. is lessening
2. lessens
3. lessened
4. has lessened

I personally would use lessens but I might be wrong? What do you think?

Answer 1

I would use "lessens" for simplicity, but I think the truly proper use of future perfect tense would be "will have lessened."

Answer 2

“By the time milk is over a week old, its drinkability has lessened.”  I believe you need to know the context: are you talking about a future event (by the time my daughter, who is now five, is in her late teens, …), a generalization of (past) observations (e.g., the milk), or something else.

Answer 3

In the context that you have presented, 'lessens' would be correct, as it is the correct present tense. However, this is because there is no context to further evaluate the statement.

If you were moving chronologically, (an earlier sentence might have been 'Children are better at retaining information than adults.'), then 'has lessened' would be correct, because you are trying to show a progression. I imagine 'has lessened' is probably what you are after, but I couldn't tell you for certain without seeing the greater context.

If you are putting the focus on the children, though, you should use 'will have lessened'. That is to say, if the sentences before and after your example are about children, then the mention of the teenage years is a digression into the future.

Answer 4

Surely they ARE in their teens so the ability to learn IS lessening. Present tense even in this slightly complex form. 'Lessens' would go better with, 'When they are in their teens...'

Answer 5

Lessens is correct in your example. I can think of a scenario when lessened would be correct. Consider:

By the time they were in their late teens, their ability to learn __.

That would be lessened.