In theory you could class them to Chf DP gunners at lvl 62 and they will do fine (the added crew growth from the classing should be enough to counter the ability loss).

However IJN has crappy AA slots on most ships till Yamato (Kongo and the PBB have workable AA tho) so you can wait more to save weight (the DP gunners will be lighter).

So just class them when they're at Yamato level. 

Classing sailor -> More men appear on sailor for every level.
More men -> Heavier

So if you class AA gunner later he will have less men.

Less men -> Lighter
Lighter -> More space for other things ammo / armor (bulge)

Classed at lvl 62 - Good
Classed later - Better (lighter)

There is a certain number of men on a lvl 1 sailor... 50 i think.

Every levelup the capacity of the sailor increases depending on his classing (for fighters it actually decreases per classing whitch is why some people decide to not class their fighters beyond "fighter pilot" at all).

Now since lvl 62 (the level you can class DP gunners to Chf.DP gunners the increase in crew capacity is high enough to compensate for the loss of ability growth (assuming you have 100 vets and near max experts).

However since IJN has no ships with good AA slots at lvl 62 most people wait and keep leveling them till they get to Yamato.

If you class them later they will be slightely lighter and since there is little use for DP Gunners for early IJN BB tiers (bad T slots) there is no loss in waiting a bit longer.

I understood everything but this part : "the increase in crew capacity is high enough to compensate for the loss of ability growth" lol,sorry. :P

Can you please repeat it in simpler English? But not that simple because I'm not that bad lol.
And I have about 30 Vets with my Arm.Sailors (who'll be Chf. DP Gunners) and 120 Experts...
I'm waiting for the Veterants event in the summer so I'll have 200% of success at rolling vets. 

Crew capacity (the number of rookies experts and veterans on a sailor) takes part in the true ability calculation.

More sailors -> More space for Experts and Vets

However

Classing as DP gunners -> Less ability growth for ACC and REL

The ability growth reduction is high only for the first classing after Armory sailor (DP Gunner) while classing to Chf DP gunner only increases sailor growth and AAW.

If you class directly from Armory sailor to chf DP gunner they should have enough crew growth to match the worse ability growth.

In short...

Just class them at lvl 90 or something.