Doesn't sound to me like anything unusual going on here! They simply have no idea what is or isn't acceptable use of their jaws or what they are capable of. They need our help to teach them.
I have to say though, I'm a little uneasy with just saying they grow out of it?
If left to fade out on its own accord, we miss the opportunity to teach them the most important life skill of all - that if they ever do feel the need to bite, they do so with no force behind it.
This is important if we accept the assumption that any dog is capable of biting or nipping if pushed to their limit or through over-excitement during play or a play-fight, we can't afford for any kind of damage to be done. As much as we think of our Labs as butter-soft cuddle bunnies that wouldn't hurt a fly, statistically they are responsible for more bites than any other breed -
... But of course those statistics are skewed because Labs are the UK's most popular breed, the odds surely increase in line with the population size. As would the proportion of dangerous ones through poor breeding, inadequate socialisation, have been attacked themselves, mistreatment or bad training.
This article is my personal favourite for how to tackle this. Feel free to substitute any words or commands as suits (for example I was already using Off for something else, this authors Off is more like my Leave It & Drop It).