Young Steve says that he is in his junior year, which would have made him 15-16 years old at the time of the flashback (1992). This is consistent with the age of Alex O'Loughlin, born August 24, 1976, meaning that he was 15-16 in 1992, and that would make Steve McGarrett 35-36 for episodes in 2012.
After Five-O charges into Nicky Chang's warehouse, Kameakona sings "Five-O Five-o what you gonna do" which is a parody of Bad Boys.
The shot of Steve's father's headstone at the cemetery stating date of death Sep. 20, 2010, is consistent with the air date of the pilot episode when his father is killed.
Jack also says to Steve "In a couple o'years you're going to be 18, further confirming that the present age of Steve would be 35-36 in January 2012.
In the flashback scene with Steve's father, Jack McGarrett, which begins with an on screen title "McGarrett home 1992," Jack tells Steve "Joe got you into the Army Navy Academy," this is a real college prep military boarding school for young men, located at 2605 Carlsbad Blvd, Carlsbad, CA 92008.