You can do Roth conversions of your Traditional IRA at any age.  You can only convert your employer sponsored plan (401(k), 403(b) etc.) once you are eligible to withdraw funds.

Yes.  Employer sponsored plans such as 401(k)s and 4013(b)s, etc, can be converted to Roth IRAs, but must first be converted to a Traditional IRA.

Unlike contribution limits and income limits on Roth contributions, there are no limits on Roth conversions.

Roth conversions must be completed by December 31.

You will receive a 1099 for the amount of your Roth conversion and the taxes must be paid by April 15.