27 is kind of  young for a Lt. Commander (O4) in my opinion. I know I was a bit older in the officer progression, I was a First Lieutenant (O2) at age 27 and promoted to Captain (O3) at age 29. The normal progression (for humans) is commissioned (O1) at age 21/22; O2 at age 23/24; O3 at age 25/26 and then promotion to O4 would come no sooner than four years later and could take longer than that. I was never promoted to Major and held the rank of Captain for just over nine years. That is the longest you can hold the O3 rank. A typical (human) should be between the age of 29 and 35. Then O5 between age 33 and 43 and O6 between age 37 and 50. But wartime promotions will ALWAYS precede these typical time lines. If you study history, you may note that promotions in the US Army between 1918 and 1940 slowed down incredibly. An officer in Grade O2 - O5 could remain in that rank for ten years or more.