Should I be worried?

Ok, this is going to sound daft but I was just reading another thread about discolouring on fur...

As you may (or may not) know, Snoop is a Black Lab, well, 3/4 Black Lab and 1/4 Alsation. Well, when he was just coming up to 2 yrs old he started to go a little grey under the chin. He's 4 this summer and it has gotten a lot worse, to the fact that people are surprised to learn how young he really is. Should I be worried about this?

Here he is at about a year and half old... no grey...

...and again recently...

I wouldn't worry, My American Bulldog girl is five in April this year and going grey, I think it's more noticeable if you have a dog thats very dark around the face

A friend of mine has got a black lab who has gone grey around the mouth and he's two - I'm sure it doesn't mean anything.....