I have 6 players. Two understand how to play. The others have been playing from between two years and four years. I think that is adequate time to get a grip on the most basic rules. I still find myself reminding them how to calculate attack bonus when it is printed on their character sheet. Even helping the figure out saves and damage. I should be collecting a check from them for the extra work beyond running the game. Any advice will be read and appreciated.
Some people may never get the rules of the game. Of course, as an old AD&D guy where the DM handled the rules and the players just had to roll the dice and tell the DM the result, I have no problem with this.
I love having new players at my table, and many times they are players that otherwise probably wouldn't play the game. Why? Because the basic rule at my table is simple: tell me (the DM) what you want to do, and I'll let you know what happens. I might ask you to roll a die or two as well.
Several of the players are always more experienced than others, and they also help with the mechanics. It's really not that big a deal.
I see it as one of the strengths of the older way of playing, because the player can focus on their character and the adventure, and not the mechanics of the game. It makes getting somebody new up and running much faster too.
One of the downsides of the DM being responsible for all the rules is that it raises the requirements for becoming a DM. Having a mix of both - some players that are also experts (often more so than the DM) as well as the DM in charge of the rules helps quite a bit in this regard. Especially from 2e through 4e when they introduced so many player options, a DM had a hard time keeping up with all of the character rules. I think 5e does a decent job of scaling this back so a DM can be well versed in 80%+ of the rules and only one or two other players really need to be up to snuff as well.
Otherwise, we make sure that the players have a well designed and completed character sheet so we can just ask them to roll a die and tell them what to reference on their character sheet to give me what I need. I generally have the majority of the PC stats handy on my side of the screen anyway, since I make extensive use of passive ability checks. It's not difficult to make a quick chart or table that includes all of the PCs for quick reference. In which point I'm back to just needing them to roll the die and tell me the result, just like I did in AD&D. If you're well organized it really doesn't take much time or detract from the game at all.
My interest as a DM lies in finding players that get into their characters, the setting, and the narrative. The rules are there primarily to help me adjudicate the action. While I love writing (and rewriting) the rules, I like them to be as invisible as possible during the course of play. Whether the players learn them or not is of much less importance to me.