"For those wondering about the best time to apply, let's look at percentage of students matriculating by age groups at time of application:
- Age 23 and younger: 10%
- Age 24-26: 78%
- Age 27-29: 12%
- Age 30+: 1%
The numbers definitely speak for themselves and probably don't need any added insight."
I recommend reading the rest of his analysis.
For any current military men or women applying to MBA programs, and HBS in particular, his blog is worth your attention.