I received a letter from my 401K that I have to use some of the money because of my age. I am still working and plan to work until 12/17. why would I have to cash it in and open another 401K? will I be penalized if I don't do it because of my age?
OK, you need to understand the concept of “Required Minimum Distributions.” No matter what kind of retirement account you have (IRA, 40l(k)) and except for Roth IRAs, you must start taking money out the year after you reach age 70-1/2. There is a prescribed schedule, designed to make your money last your expected lifetime. Those withdrawals are taxed as ordinary income, because the government wants its share! (Remember, your contributions were tax deductible when you made them; now the govt wants taxes on that money and any growth over the years!)
Are you still working for the same company that holds your IRA? If so, the benefits department will guide you to the appropriate withdrawals. Go in and discuss — and be sure to let them know if you have any other retirement plans. The withdrawal can come from just one plan, but all balances must be considered to determine the appropriate amount.
If you are not working for the company that holds your 40l(k) I suggest you do a direct rollover to Vanguard or Fidelity, and they will advise on withdrawals. But since you have reached this age, they might require a withdrawal to be made before a rollover can be accomplished.