Are you asking how old the dog should be before the pinch is used as a mode of correction?

 Quote: Ross Rapoport

How old is old enough for a prong?

Ross, if you are asking at what age is it okay to introduce the prong, Ed's opinion is between 4 - 6 months.

Replies above are pretty good...

Talking from a pet perspective here...

You mentioned your dog stops and sniffs, I'm assuming it's not pulling you forward to sniff...but rather 'lagging behind' and you want him to 'keep up with you'...

I've had/worked with dogs like that I would either:
1. get a long line, just keep walking (dont wait for him) and let him catch you up (i.e. u don't do anything to him) or
2. the line will eventually run out, and he'll be pulled in your direction.
3. if I think he's 'mature' enough or if I've had enough of his time wasting tree sniffing crap; I'd teach him how to walk properly when I say so.

Refer to your first reply which I hope I'm right in saying is 'Koehler' style for walking next to me (basically the dog avoids corrections by staying near me + praise when the dog is by my leg)

It can help by clipping a long line (for releasing) and changing it to a short 6ft line when you want the dog to 'heel'. The sound of the 'click' can signal to the dog the change of the leash better than just 'saying' heel/ok/free etc. also helps the human, cuz we can be dumb sometimes (maybe just me) and forget if I've told the dog to heel or not or what.

BUT I don't ask my dogs to walk 45 mins next to me when they are 5 months old and have no idea what I want.

Maybe they walk 1 minute next to me when I first teach them the exercise, then as a reward they're released either off-leash or on a long line. The next day 2 mins, then 3, then 5 then 10 etc.

Also, teach the dog in super LOW distraction environments first + training him in the same place helps them learn too. (place orientation)

I think the replies above explained most of it but timing is very important or you just end up jerking the dog around for no apparent reason, or so your dog might think.