Hello, Welcome to Just Answer.
It's wonderful that you are still working and want to contribute to your 401k.
You may still make contributions after age 70 1/2 if you are earning income with this company.
The 401k is a qualified plan that normally requires minimum distributions after age 70 1/2, but the exception to this is that if you are still working, you don't have to take the distributions until April of the year after you retire (unless you were an owner of at least 5% of the company that you work for.)
Here is a link to information about required minimum distributions for your reference:
Sorry for the delay, I was researching your question.
According to Publication 575 (http://www.irs.gov/pub/irs-pdf/p575.pdf) , and I also checked with an IRS representative,
You must begin to take distributions from your 401k plan with firm A by April 1 of the year that follows the later of:
The calendar year in which you reach age 70 1/2, or
The calendar year in which you retire from employment with the employer maintaning the plan
Even though you are still working, you have separated from service from firm A, so you are required to start taking distributions from that particular plan.
Regarding firm B, Publication 575 also says that a certain plan may have requirements for distributions after age 70 1/2, even if you are still employed, so I would check with the plan administrator to make sure that you are not required to take minimum distributions from the new firm's plan.
This information is contained on page #31 of the publication that I gave you the link for.
Let me know if you have any other questions.