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.