I wish I’d been able to understand Kipper from the off, but I’d say at just over 2 I’m starting to get a handle on him. I’ve found the pressure to know what’s best for him as an individual quite difficult, so wish I’d known early on to take it easy, not to feel pressured to do too much too soon.
You can't expect anyone to understand the nuances of dog communication from the get-go. It's just not reasonable. That is why I think it is far better to err on the side of caution with those early experiences. You'll do a whole lot more damage from not understanding a sensitive dog's needs than you will from under-exposing (if there is even such a thing) a robust dog. And it can be really difficult to even recognise what a sensitive dog is - Willow seemed really bold to me as a puppy, but there was a lot that I didn't see because I didn't have the experience. As others have said, whilst there is certainly a window of early learning (we know this from the way the physiology of how brains develop), learning does not stop once that closes. It is not "now or never" as so often we are led to believe.
When it comes to training, you have to have reasonable expectations, too. As a young puppy, when Squidge was on lead, I could ask her to sit and watch people go by and not interact with them, but off lead on a beach - hell no! If I saw other dogs coming, I would assess their body language and decide if I wanted to just let her run over and say hi, or if I would stop, put her on lead, and ask the owner first. You have to make those judgements, and it takes practice to do so. Up to last year, she would still run to other dogs very often if I didn't catch sight of them in time. She always had a "range" and if they were outside of that she would stop and I could recall her, but that's her - I wouldn't expect all dogs to be the same. Now, she will generally stay with me until I tell her she can go, but she's over three years of age. Granted, we don't have the same opportunities to practice as many of you in the UK, and maybe she would have picked it up sooner over there. Or maybe not. Maybe it just needed the maturity. This is stuff I just don't stress about these days; there is no timescale. Things you practice more become stronger faster. Things that are more difficult for the dog are going to take longer. Every dog is different. Even now, I know that in a new environment, she will regress and won't be able to control herself. When we come to the UK, she will be an excited mess for the first while. That's OK; I expect and understand it now.
What I won't do again is allow people to pet my puppy while I'm holding him/her. I did that when they were still being carried, but I wouldn't do it now. The puppy has no control, very limited options to say no. Ugh, it makes me shudder to think that I allowed it at all. For Squidge, there were no obvious repercussions. Were there with Willow? I can't say, of course, but I wonder what I would be able to see if I were able to watch videos of those interactions now. Nope; for me, it's important that the puppy is able to control all interactions with the other person/dog at their pace.