Sign of aging?

I've noticed that Monet, who just turned 6, now has some white fur along the back of his neck. It lines his collar.

I've never had any aging cat as Monet is my first. I am not worried or anything, I am more curious to why he's got white fur where he's never had before. Aging? The collar since it's along the collar area only? at what age do cats become seniors and start to show physical signs of aging?

Here's a photo. I've moved the collar up a bit to see the white fur better

That's an interesting question!  I've noticed that our tuxedo, Ozzy, is getting more white hairs in with the black.  He's almost 6.  I found this thread:  http://www.thecatsite.com/t/227504/black-cat-turning-white

It sounds like our kitties can get 'gray-haired', too, like we do.  

I hadn't thought about it with cats.  I know dogs go gray, with dogs it usually starts on the muzzle and spreads.  I would guess that it happens in some form with kitties as well.  I think it's a subjective thing as to when kitties are considered seniors.  I know with my kitty that doesn't have a chronic condition the vet recommended annual blood work when she was 10.  She didn't say because she was a senior, I just assumed that was why,

Hannah is now 10 yrs old and I've noticed the area around her mouth and nose is getting lighter.  She's always had lighter hair there, but it's even lighter and whiter now.

Mogli will be three in June and is getting single white hairs along his shoulders. The vet said it's quite common (and more noticeable) with black cats.

Yes! My friend's cat (who lived to the ripe old age of 23) started out in life totally black but as she got older there appeared a smattering of white hairs here and there. Isn't it interesting how there are signs of ageing which are common to all species?