As late as possible. I'd say at level 80 or after.

I classed mine at 65, and I regret that.

There's an infinite answer. You information is not sufficiant.

You should provide answer such as what base, are they going to be boosted, how much vets as your goal? You're going to max experts them?

On the KISS principle...as late as you can as mentioned...now just a thought....what about lvllng a pair of +14 boosted hero sailors....some might say wasteful ...Im thinking ..has someone tried it.......

to be really honest uk AA isnt good, so late class them as late as possible

I currently run lvl 115 AA Gunners on my QV.   Elite reload left Arm. Sailor and classed full once the total displayed ability of reload reached 1100, then I boosted them.  Currently they have full experts and 170 Veterans.  Using the 5.25" dual AA guns on the QV I have the gun's reload cap in Great Battles.  In World at War the rate of fire is significantly higher and the damage per shell is above average AA wise.  Feel free to PM me ingame if you want a demonstration.

I play AA with UK line and recently reach L2. I class them  on time and have to say they worked fine. Now they have over 110 vets and almost full experts.

Personally I´m a strong defender that every BB shuold have AA equipment for self defense and team collaboration when a BB is repairing back in the battle line, to support the rest of the team.

 

What about non AA gunners, like acc huge (late class at heavy). Still using emerald and need to train a lot of support sailors while also training my gunner (bought as 10/12 acc heavy). I'm using 5.25"/50 RP 10 Mark I D ( I think) because I found other lower calibre guns have longer reload. So is it the right gun choise cuz I'm having quite a slow reload.

UK AA is by far the worst of the original 4 nations. So don't expect really fast reload guns (unless using Pom-Poms). I use reload gunners and 5.25 RP I guns. I get decent results, not as good as US or KM. My biggest complaint about UK AA is not the reload, but the weight of the guns. The UK AA guns (excluding Pom-Poms) weigh twice as much as US counterpart, taking up displacement that could be use for speed / bulge. There is a reason why so few people use AA with UK ships. They're just not that good compare to other nations.