Question 1122465
This is a sample.  The population itself is not defined, but it if is anything larger than this sample, which it likely is, then the population mean is unknown.

The best estimate is 11.5, x-bar is 11.5, s (not sigma) is 2.7, and SE is 0.54
90% CI is +/-t df=24 *SE
=+/-1.711*2.7=+/-4.62
mean +/-4.62 
(6.88, 16.12) units are years