Darwin Finally Became Assertive... When Did Your Doodle?

So during lunchtime today we took Darwin out for a romp in a large grassy area near our home. There is almost never anyone there, although we've seen other dogs playing there on rare occasions. Anyways, we let Darwin off and as he was playing he was joined by 3 other dogs - 2 Malamutes and a Husky.

It went really well, they all got along and had a fun time. However, we noticed something different. The Husky puppy went to mount Darwin, and Darwin turned, growled, and made a snapping motion. The puppy got down and they kept playing as though nothing had happened. The puppy didn't mount him again.

I found this odd because he has never done anything even remotely like this when playing with another dog, even when they mounted him (which believe me used to happen all of the time). I wondered if it could be that now he's getting older, he's more assertive or less tolerant of that kind of behavior? Don't get me wrong - I am happy that he's learned to stand up for himself, and say no to behavior he's not enjoying. Just a break from the norm I guess.

I also noticed that he was sticking close to us. Every few minutes he'd come back and sit against our legs, then run back off to play more. The other dogs weren't badly behaved or overwhelming at all, and he didn't act stressed whatsoever.I was just wondering if all of this might be a sign that he's transitioning from a puppy-ish play style to an older more mature one.

It has been quite a while since he's played with unfamiliar dogs... maybe 6 months or more.

Did your doodle change in play style from puppy to adult? Did they like playing with other dogs less as they grew older? I think I remember someone saying that their dog (Was it Adina's Rosco?) liked playing with dogs less when he got older.

Thanks everyone! Here are some pictures from the outing.