There is no age limit for contributions into the Simplified Employee Pension plan (SEP)
As long as otherwise the person is eligible for such contribution - he/she may continue such contribution after attaining the age 70 1/2.
An eligible employee is an employee who:
Moreover as long as an employee earns at least $500 for the year (in additional to tests above) he/she MUST be allowed to participate in the plan.
Please see for reference IRS publication 560 - http://www.irs.gov/pub/irs-pdf/p560.pdf