If you begin investing at age 25 instead of age 20, you need to invest $120 more per month to have $1M at retirement. If you begin investing at age 45 instead of age 40, you need to invest $718 more. Why is the difference between 40 and 45 so much greater than the difference between 20 and 2

Due to less time in retirement.

Explanation:

The difference between 40 and 45 is much greater than the difference between 20 and 25 because there is less time remaining in the retirement. In 20 to 25 age, more years are remaining for the retirement i.e. 35 to 40 years while on the other hand, in 40 to 45 age, less number of years are remaining to retirement which is about 15 to 20 years so that's why the person with lower age less money per month as compared to higher age person to reach the same amount.