What is the Lower Bound of the confidence interval?

Question

In a sample of 80 children, the mean age at which they first began to combine words was 16.51 months with a sample standard deviation of 9.59 months. 

Construct a 95% confidence interval for the mean age children begin to combine words

Answer 1

Only the SAMPLE standard deviation was given; therefore a Confidence Interval for t was used. The 95% Confidence Interval, using the t-distribution, is (14.38 months, 18.64 months)