Distributions from an IRA must start the beginning of the year you turn 70 1/2. They are generally based on your IRA balance divided by your life expectancy, either singly or jointly with your IRA beneficiary. IRAs are meant to provide for retirement, so if you don't take your required distributions, you'll be subject to a substantial penalty.
Contributions made by an employer through a retirement plan known as a simplified employee pension (SEP) are actually contributed to a Traditional IRA, and they can be combined with regular IRA contributions. Effective in 2002, after-tax assets from a "qualified retirement plan" and assets held in governmental 457 plans are eligible for rollover to a Traditional IRA. To protect the option of someday moving them to another employer plan, such assets are often best kept in a separate IRA that contains no other contributions.