I don't have a great answer but there is a moment in DH where Harry's hearing about how attending is mandatory and that it's a mechanism for finding MB students and he imagines 11-year-olds poring over new books only to find out that they won't ever actually see Hogwarts. Now, Harry has no idea whether any 11-year-olds have actually gotten a letter or not so this isn't definitive, but maybe JKR included this because she believed it to be what happened??
The letter thing is remarkably inconsistent because you should probably get your letter in mid-July with all the materials and stuff, since it asks for a reply by July 31, and Harry started getting his on the 24th or so, but Pottermore says that McGonagall got her letter on her birthday, and she was born in October. October is way before Dumbledore died. So it's unclear when you actually get your letter and that might impact whether mb students were notified as usual.
My personal feeling is that the MB registration commission would probably have wanted to round up 11 year olds too. I don't see why they would go easy on 11 year olds and throw 12 year olds in jail, so I imagine them sending a ministry official as usual, and then like, interviewing them when they're supposed to go to Diagon Alley and get all their stuff and using that as an efficient trap. But it's obviously up to the admins since canon is unclear both on what happened to first years and when letters come. Maybe Harry's just came in mid-July because otherwise he wouldn't have time to reply before his birthday.