Turbotax is wrong based on IRS PUB 503 because age is determined at the time services were performed. However, there are other requirement which must be met. Review PUB 503 flowchart on page 5
https://www.irs.gov/pub/irs-pdf/p503.pdf
if you qualify and properly answered all the questions then it's a Turbotax bug which only Turbotax can fix.
IRS REG 1.21-1(a)(4)
The requirements of section 21 and §§ 1.21–1 through 1.21–4 are applied at the time the services are performed, regardless of when the expenses are paid.
Who Is a Qualifying Person?
Your child and dependent care expenses must be for the care of one or more qualifying persons.
A qualifying person is:
1. Your qualifying child who is your dependent and who was under age 13 when the care was provided
