At some age you are correct, but I don't think 5 or 7 is it. Kid's reasoning skills just aren't developed enough yet at 5 to really understand what advertisers' motives are. Exposure will do little more than desensitize them to ads and instead let their subconscious absorb the message.

I'd rather my kids find ads jarring when they are exposed en masse to them rather than be too comfortable around them.

There are a lot of things that 5 and 7 year olds shouldn't be exposed to yet, not just ads.