Enuresis clinics will usually take at 7yo, but slightly earlier if there is an interesting co-morbid or any daytime wetting.
See, the boy with the cousin I'd see as a bog standard example of how and why an alarm works...
Two reasons for bed wetting, deep sleep and/ or non production of vasopressin.
So if a child is a light or normal sleeper but body is not mature (doesn't produce hormone) then the cure is a synthetic hormone in the form of desmo.
If a child sleeps too deeply to be roused even when they are leaking, an enuresis alarm will be needed to rouse the child. For really deep sleepers, of course, it is the parents who need to be roused by the alarm, and then to move into full on deeply unpleasant awakening mode - clapping, shaking, lights on, (shouting if necessary) so that the child connects leaking to an unpleasant occurrence. Then the subconscious reacts by hearing the alarm (instead of the full on deeply unpleasant parental hoo ha), and eventually, the noisy unpleasant alarm every time there is a leak means the body makes the unconscious connection and rouses the child before they leak, so they stumble to the loo and pee in the peace and quiet, instead of the intrusive wailing and buzzing of the alarm, or the parents... So, in my opinion, the boy whose cousin came to stay wasn't controlling the bed wetting deliberately, he was just sleeping lighter because of his cousin and his fear that the alarm would go off. No real 'control' about it at all - just a really deep sleeper whose subconscious worked pretty fast when there was an impetus. 
On that note, I'm unsure why the op has decided to try alarm first rather than desmo. In my honest opinion, the hideousness that is an enuresis alarm should be the case of last resort as it is so unpleasant for the entire family, and works over a period of weeks, which are characterised by disrupted sleep for everyone. And, to be blunt, if deep sleep isn't the problem, then it isn't going to help. Really you should rule out the simple hormonal fix first. If desmo works, it's easy and problem free, and there are no weeks of sleepless nights for the entire family, just pretty immediate success.
I know people are often anti medication, but tbh I don't get it. Why would you put your family through weeks of sleepless nights when it might not help, when a week on desmo with everyone sleeping will tell you whether that's the fix?
Of course, you also get kids who don't produce the hormone and are deep sleepers. They can be a bit tricksier.
Unsure why no sleepovers though. I run youth activities (cubs, brownies etc) and we always have kids in pull - ups and bed wetting on camps. And private sleepovers too. Kids are less likely to wet at camp for the same reason as the boy above (their subconscious is on total full alert) but tbh it's all dealt with easily and quickly. Bags for the pull -ups, spare sleeping bags, and making sure the campsite has a shower for use if necessary. No bother. And if it's a sleepover at a parents, just let the parent know and make sure Ds has a plastic bag and spare pjs. No need to punish the kid for something he can't help by not letting him do perfectly ordinary 8yo stuff. He needs reassuring that loads of boys wet the bed, and that it will be easy to deal with on a sleepover. Or try some desmo. 