Is there an official standard age for when children USUALLY start दिखा रहा है signs of magic?

I don't think there's a specific standard age. It's like walking या speaking for a toddler. There's probably a span of time during which the first signs should manifest themselves indeed, like when a child begins walking या speaking, but that's not defined.

Not दिखा रहा है these signs at a दिया age doesn't mean there's a problem. Of course, from a certain age on, parents may start worrying, if द्वारा 18 months या 2-3 years, the child doesn't walk या speak, respectively. I guess for magic, in the HP universe, that's the same thing, but most likely to occur between age 2-3 to something like 8-9 (that's just how I see the thing). Hence the harsh way Uncle Algie dealt with the issue regarding his nephew Neville.

Obviously, Lily and Severus could do magic, in quite a controlled way, before they met. They met when they were 9. I suppose they were in the norm in terms of age. Yet, whenever their first signs of magic happened, that did not prevent them from being powerful and gifted witch and wizard. Besides, training has a lot to do with it.

Let's remember that kids are all different. There are slow learners and fast learners. There are slow developpers and fast developpers. But eventually, kids will walk, speak and - in the HP universe - दिखाना magic. Neville is a good example thereof.

At last, kids have to दिखाना signs of magic before attending Hogwarts. Otherwise, they would not be able to follow the courses. That's probably why magic is trained at high school/secondary level only, not before, just to give time to children's magic to mature enough to allow them to follow the teachings at Hogwarts.

I don't think there's an official standard age. It was implied in SS that Harry and Ron had been दिखा रहा है signs of magic for a while, but Hermione had not. (Her family had been completely surprised when she got her letter saying she was a witch, and if she'd been doing some of the things Harry had been doing for a long time, they would have at least suspected there was something odd about her.) In GoF, there was a little girl (I think they कहा she was three या so?) that turned her father's wand into a slug. However, I think if they haven't shown any signs द्वारा age seven या so, it's worrisome.

In my version of chapter seven in DH, there is no mention about children doing magic at all. (I don't even know why there would be in any other version, as that's the chapter with Harry's birthday and where the trio get the things Dumbledore left them in his will. There really isn't any place to allow a conversation about children doing magic at a certain age.)