Timeline

At age 50 – Begin making catch-up contributions, an extra amount that those over 50 can add, to 401(k) and other retirement accounts.
At 59½ – No more tax penalties on withdrawals from retirement accounts, but leaving money in means more time for it to grow.
At 62 – The minimum age to receive Social Security benefits, but delaying means a bigger monthly benefit.
At 65 – Eligible for Medicare.
At 66 – Eligible for Social Security benefits if born between 1943 and 1954.
At 70½ – Start taking minimum withdrawals from most retirement accounts by this age; otherwise, you may be charged heavy tax penalties in the future.

Source: http://www.dol.gov/ebsa/publications/nearretirement.html