If you invest $1 a day starting the day you are born and earn an annual 9% return on investment (ROI), you will reach $1 million at age 64 with an out-of-pocket investment of $23,376! If you invest $4,026 on the day you are born and earn a 9% ROI, you will also reach $1 million at age 64 with much less out of pocket. If you wait until 21 to start investing and want to reach $1 million at age 45, you will need to invest $1000 a month. Bottom line, if you want to become a millionaire, it's much easier to start saving a small amount at an early age than to make up for lost years.
Time in the market is more important than timing the market.
In order to maintain your lifestyle, you will need about 70% of your pre-retirement income. Keep in mind that your expenses may be lower or higher in retirement because you have fewer expenses associated with a job and you may have more expenses for things like travel and recreation. Withdrawing 4% of your nest egg has been a safe withdrawal rate in the past, however it depends on how you manage your expenses and your asset allocation.
You can use this calculator to estimate how much your nest egg could be worth in the future.