I think by design it is supposed to be about 2 years between finishing command and board. That is shrinking - lots of guys in command close to twenty years and guys are going to O6 boards at the 20 year mark.Follow-on question: how many months buffer is usually built-in by the PERS wizards between the end of a CO tour and the URL O-6 board?
Early O-5 + on-time CMD screen could be bad if you don't get that last CO FITREP and/or start a joint tour before your first CAPT look.
Agree with previous posts that an early O5 could be an issue - you could end up as an O6 select at high water mark for your CO tour. Similar issue happened to some folks that early selected O4 years ago and didn't have the paper (due to time in squadron) when they went to the O5 board.