When Do I Have to Withdraw an Inherited 401(k)?

No. A distributee other than the employee or the surviving spouse may not roll over a plan distribution. The Code instead provides a direct trustee-to-trustee transfer to an IRA established to receive it, which is then treated as an inherited IRA subject to the post-death rules.

April 1 of the calendar year following the later of the year the employee reached the applicable age and the year the employee retired, unless the employee was a 5-percent owner of the employer. For an IRA the retirement prong does not apply. The applicable age depends on the owner's date of birth.

Yes. A distribution from an employer plan paid to the surviving spouse after the employee's death is treated for the rollover rules as if the spouse were the employee. The statute also lets a surviving spouse elect to be treated as the employee; the regulation writes that election as a plan provision and reserves its mechanics, and a proposed rule to fill them in was pending when these rules were verified.

The Code applies requirements similar to section 401(a)(9) to 403(b) annuity contracts and custodial accounts, and applies section 401(a)(9) directly to eligible 457(b) plans. The inputs here treat those accounts as employer plan accounts.

25% of the amount that should have been withdrawn but was not, paid by the beneficiary. The rate drops to 10% when the missed amount is withdrawn from the same plan and a return reflecting the tax is filed, both inside the correction window, which ends at the earliest of a notice of deficiency, an assessment, or the last day of the second taxable year after the year the tax applies to. The IRS may waive the tax for reasonable error on a Form 5329 request, and the regulation waives it automatically in two cases.