I have yet to participate in a raid battle and see what it's actually like because my data ran out :( but if it's similar to regular gym battles just with the difficulty ramped up, I get the feeling players of a lower level will naturally be discouraged by the difficulty. I certainly went and tried to participate in gyms the moment I hit lvl 5, but got overwhelmed and discouraged pretty quickly, and didn't try again until I had some pokemon over 1000cp.
That said, I think the lowest limit should be be lvl 10 or lvl 15 at the very least, maybe even 20 or 25 - purely in terms of game progression, I don't think it makes sense to have access to what is essentially a boss battle the moment you've levelled up enough to participate in any battle period. It would be like if you could challenge the E4/champion in the main series games before collecting any gym badges.
Actually, now that I'm thinking about that, maybe it would make the most sense if you were granted access after winning a certain number of gym battles - that way the people who can access the raids are the ones who both have pokemon of a high enough level AND actually know what they're doing. (*side note, I'm not saying this as an elitist player looking down on people who are new to the game or not as good at it, I'm saying this as a lvl 24 player who has not yet won a single gym battle because I've only had the chance to give it a go a handful of times, and would be absolutely obliterated in a raid battle no matter what level my pokemon were at)
Its hard to say in my area no one seems to be participating in the raid battles I have been around the gyms no one is standing around. I have tried doong them on my own but obviously only level 1. There was level 3 and 4 the other day and no one attempted them!
Yet the gyms themselves are very active indeed but the raid battles are not.