It depends--are you looking at a K-12 or K-8. A K-8 will give you a leg up in high school private admissions as well as more focus on your kid during middle school. Fifth is actually a good entry year for K-8. That's when upper school starts for most. The K-12 schools will vary by school.

Your best bet is to look at the websites of the schools. In other words, do the homework.

Most of the schools have entry in 6th grade (the ones that are 6-12 are, of course, easier than K-12s). Some grow is 7th, all grow somewhat in 9th. Beyond that, entry is nearly impossible unless you are coming from out of town or have a very recognizable last name.

Most of the schools are very competitive. You will see it debated extensively here. The less competitive ones are still quite competitive, so apply wide and have a plan B (which is sounds like you do).

Pretty simple:
6th or 7th.
8th is not an entry year.

9th is hard... if you know you want private try in 6th or 7th. If you don't get in (many don't) then you'll have a slight edge as a repeat applicant for 9th.

We are keeping our kid in DCPS until 5th and then thinking of private starting sometime between 6th and 9th grade. What are the best entry years for DMV privates? Our kid is a typically developing slightly above grade level kid (not super gifted, but does well in school). I'd love to understand more about the best entry grades, both in terms of ability to get in and good entry grades from a social standpoint too. We are zoned for Deal, and I think our kid would be OK there, but ultimately would do better in a smaller environment, so we anticipate private either at some point in middle school or for high school. We are pretty flexible about when she starts (i.e. any time between 6th and 9th), so we're looking for advice about when it makes sense to make the transition.

We are new to DC, so don't know the lay of the land for privates yet. All advice is appreciated!