Yes. A 401k including a self-directed solo 401k can require the employee to reach age 21 before he or she may participate in the plan (i.e., make contributions as well as to transfer other retirement funds including IRAs and former employer plans to the solo 401k plan). I.R.C. 410(a)(1)(A)(i)
Once the W-2 employee reaches age 21, he or she will need to be an owner and an employee of the self-employed business in order to participate in the solo 401k plan. Therefore, if the employee who is 21 and not also an owner and employee in the business, the solo 401k plan will need to be restated to a full-time employer 401k if the self-employed business wants to continue to sponsor a 401k. Another option is to close the solo 401k and transfer the funds to an IRA. Essentially, once the self-employed business employs a W-2 employee who is age 21 or older and who is NOT an owner in the self-employed business and works 1,000 hours or more in the business, the business can no longer sponsor a solo 401k plan. There is also the three year part-time employee 500 hour rule under the SECURE Act that applies to non-owner employees who are age 21 or older. To learn more about the 3 (three year-part time rule) visit HERE. Note: Starting in 2025, the SECURE 2.0 Act reduces the three year consecutive requirement to two years of 500 hours.
